Wave 1 CBT Rough Guide:
I'm gonna be looking through the CBT popfile and trying to write rough guides for the first couple waves.
Personally, I've only completed the first 3 waves, having beaten wave 1 twice and waves 2 and 3 once.
I am a CBT newbie, but I've found it extremely fun and (in my opinion) winning is not a matter of mechanical skill, but a matter of planning and teamwork.
Having said that, the main issue of the first wave is the lack of credits. 10 credits allows you to buy one teleport to spawn canteen (which you never should, lmao) and nothing else.
This means staples of MvM such as projectile shield, explosive headshot sniper or gas passer pyro are not available.
Additionally, every single wave 1 robot is one which needs a sightline to you in order to shoot you. Soldiers have splash damage, but they aren't intelligent enough to use it to shoot you behind corners or anything. The hipfire snipers do enough damage to one shot everyone who isn't behind a stock or vaccinator uber, and the brass beast heavies have an extra 50% damage bonus on top of what is already the highest DPS weapon in the game.
Thus, the answer is to abuse the most powerful combo of unupgraded weapons in the game: the Scottish Resistance and the Kritzkrieg. The sheer number of crit stickies you can lay with this combo is absurd. Quite frankly, I believe you could beat the wave entirely with nothing but enough sticky piles prepared before the round starts with multiple ubers, but unfortunately that kind of perfect play probably isn't reasonable to ask from a group of players with random and differing amounts of skill.
The fact that you do not need to be looking at the robots to decimate them with the Scottish Resistance, and the fact that crit stickies are just simply so absurdly powerful that they can one shot the hipfire + uber med comboes, makes them absolutely invaluable in this mission. At the start, have the demo lay his stickies in singular piles lined up across the path on the far side of the rock. The comboes walk this way 99% of the time, I have only seen them come on the close side of the rock once. (btw, if you were playing with me, you might have noticed I was placing piles with 2 stickies on wave 1 - I believe this is unnecessary, as a crit sticky even with maximum explosive falloff according to the wiki will still do over 150 damage, killing the medics and severely wounding the snipers - although keep in mind I am typing this having not actually tried it, this is technically a theory). There being 12 sniper + med comboes, in theory the 14 crit stickies which can be detonated by the demoman should be enough to kill these bots. Uber meds getting through are probably the most likely cause of loss on this wave, as robots can deploy the bomb while invulnerable.
That being said, I've only spoken about the strategy of the demoman (and somewhat the medic) and the strategy for dealing with one set of bots. For the rest of the team, at the beginning, your most important goal is to NOT shoot the uber meds or the snipers. If the demo dies or runs out of traps, go wild, as popping their ubers early will be beneficial, but shooting them before the demo can kill them with his traps ruins all the pre-wave planning and can easily spiral into a loss.
After the first 6 sets of sniper + med combos spawn, 2 groups of 9 soldiers will spawn at 15 and 30 seconds into the wave, then additional groups of 6 soldiers will spawn at 40 and 55 seconds into the wave. These soldiers can overlap with the sniper + med combos, which can be both a blessing and a curse. They can often die to the demo's traps, but your team will be inclined to shoot them, which could also damage the meds and pop their ubers. For this reason, I suggest either a second demo with his own traps to deal with these soldiers, or the rest of the team (such as heavies, soldiers, engineers, vacc medics and others protecting the choke - will go into more detail about the rest of the team later) shoot the soldiers when they come around the right side of the rock, which is most likely beyond the place where the demo has traps.
The largest offensive threat of the wave are the 3 Giant Brass Beast heavies. These spawn at 30 seconds, 50 seconds and 70 seconds. As mentioned before, they have a 50% damage boost instead of a 20% one. However, they also have a 300% spin up time instead of a 150% spin up time. This means that once they aggro onto someone in their vision, it will take them 2.6 seconds to begin shooting. Using this, you can move in and out of their sight to cause them to rev and unrev their weapon repeatedly while doing damage. This is not only a safe way of doing damage, but will slow them down as they attempt to rev their gun repeatedly. They have 5000 health, but using kritz + vacc on a brass beast heavy of our own will give them 1235 DPS and enough resistance + healing to kill the giants before they can do much of anything. Knockback mechanics can be effective against them as they move EXTREMELY slowly, especially when revved up, and thus have difficulty delivering the bomb. For this reason, I recommend Loose Cannon as the demo's primary, it has the highest DPS of any primary when properly distanced but more important the knockback is EXTREMELY useful for the objective.
The threat which appears last but is still extremely dangerous to the objective are the medium-sized shortstop scouts. Medium-sized is an uncommon attributes for scouts to have, but it means they have around 1000 health but still become substantially slowed while holding the bomb, unlike a super scout. There are 16 of these, and they first spawn at 70 seconds into the wave, with each additional scout spawning just 5 seconds after. They aren't too offensively threatening, but on a wave without any resistances, they can still get some kills, so don't get too cocky. Even so, as long as you pay attention to them, they can be dealt with. They get chewed through like butter by a point-blank brass beast heavy, they get slowed to a crawl with a natascha heavy, and they're fully affected by knockback from sources such as the loose cannon. There will be a lot of them, but the team can manage.
As a final note about robots, this wave has NO support, which INCLUDES sentry busters. This might sound like a blessing, but it means medics cannot get easy uber from the sentry busters. However, if a brass beast heavy is close enough to a corner or wall, the medic can use them to get ubersaws due to the extremely slow spinup time.
Now onto team comp. I've already thrown out a good number of suggestions, but I'll start with the two which I believe to be the more vital and will continue shilling: Scottish Resistance + Loose Cannon demo and Kritzkrieg medic. I cannot overstate the utility and burst damage of this demo, and the insane levels he can achieve when paired with crits. Additionally, the kritzkrieg doesn't have to pocket the demo the entire game or anything, just kritzing his traps during the preround will benefit your team extremely. The next vital team member is an engineer. Two engies is better than one, but one is functional. With no ammo canteens or reserve upgrades, a dispenser is vital for the combat classes to stay in the fight, and a wrangled sentry's 648 health is exceptionally powerful and can bodyblock. If playing with two engies, put a sentry on both corners of the choke towards the base side. If playing with just one, pay attention to the blue signs which display where the robots are running, and put it on the corner which they will be on. Sentries are excellent crowd control, but in this case, they're more useful for bodyblocking than DPS. Heavies and other combat classes will do more damage than an unwrangled sentry can, so a wrangled sentry keeping the bots in one easy-to-shoot place will improve the team's DPS more. For this reason, keep the sentry wrangled pretty much all the time. As much as it might be scary to have your sentry get healed less from your wrench swings, it's really the same healing but on a 648 HP pool with a third of the metal cost. For the next most important class, a scout is absolutely VITAL. With no money for resistances or uber canteens, no other class can go in and get the money. Use Bonk!, end of story. Without resistances, mad milk healing is not going to save anyone from a 500 damage machina shot, 30 soldiers worth of spam, or a 150% damage brass beast. Collecting credits is more important than team support on this wave. Without perfect credit collection on wave 1, it's basically pointless to go on with the mission. The final option for a support class is a vacc medic. Considering only 2/3 damage types are on this wave, and the soldiers often die quickly, the bullet res on this wave is invaluable. It's the only real way to survive the bb giants' direct fire or a hipfire sniper shot. Both types of medics pocketing a target is a real option, as a vacc + kritz BB heavy is more than enough to deal with one of the robot BBs.
Moving onto damage classes, I already mentioned demo, but stacking demos is viable as well. Having 28 crit stickies before the wave starts and using them intelligently can wipe out all the small bots that spawn in the entire wave. However, double demoing requires good planning and communication so both demos don't activate at the same time redundantly. Considering this is a server where people basically only use text chat, this can be extremely difficult. In addition to stickies, demo can also use loose cannon as an extremely powerful utility and movement option. I've done cannon jumps across the entire map to get back to the bomb hole and stop a scout from delivering the bomb with KB. I cannot stress how good this primary is. Moving onto non-demoman classes, heavy is the obvious standout. Damage scout is basically impossible, pyro has no gas and dies extremely quickly to even the shortstop scouts, sniper has no explosive headshot, and spy is even more of a meme than usual. Heavy does the highest DPS, simple as. He also has a large health pool and gets resistance while low from brass beast. Demo is the king of burst damage, and heavy is the king of DPS. Which leaves one more class to discuss: Soldier. I've seen CBT team comps where 4/10 people pick soldier wave 1. To be frank, even using his best options, soldier just doesn't stack up without upgrades. First, you get no rocket specialist, which is basically required for soldier to be useful in MvM and is what any reasonable soldier player buys first. With no stun utility and having to deal with falloff, your performance is severely gimped. You also have to deal with no clip upgrades and no reload upgrades, which means you'll be spending a fuck ton of time reloading. There are no tanks, so bison soldier isn't gonna do anything special, and the banners, while good, just aren't good enough. Backup and conch don't provide enough survivability to outlast any of the spam and bullets on this wave, and while buff banner is good, it doesn't assist sentries, and giving your heavy a 35% damage boost is not as good as the 100% damage boost you could create by mirroring him. Maybe if your team comp has more than 3 other damage classes, you can run one soldier for buff, but stacking him on wave 1 is just not very effective. His best use is his mobility, as he can chase scouts with the bomb that get past defenses, but without rocket specialist stun, there's not much he can do once he gets to them. It would be better to be a demo who can jump to the bomb hole and lay a trap there, or be the extra heavy who killbinds so he can respawn and meet the scout at the hole.
Next, loadouts/playstyles. For scout, use Bonk!. Nothing else matters too much, but soda popper is the best damaging option in case you want to throw a couple pellets of chip damage at some robots. Don't play too aggressively, you need to stay alive to get credits. Soldier, if you insist on playing him, should run beggar's and buff banner. Beggar's is the best rocket launcher in MvM and it's not even close. If you really can't handle beggar's, Air Strike is going to be better than stock if you can survive to collect heads, but stock is the best if you can't (which is probably the case). Pyro is a meme on wave 1, but use stock flamethrower for the most airblast spam, as saving the bomb is your best use. Demo should use Scottish Resistance and Loose Cannon, optimally. Please use them. Buuuut, if you really can't bare to deal with Scottish Resistance traps, use stock stickybomb launcher. It's really not bad, you can do lots of damage to the groups of soldiers and the heavies without needing a sightline to them. You can also lay traps to kill uber medics a lot, LOT faster. I would recommend it wholeheartedly if the Scottish Resistance wasn't the best unupgraded weapon in the game. Use the Loose Cannon even if you think you can't understand double donks, spamming KB is going to be a more useful utility than pipe damage, especially when your secondary puts out the absurd damage that it does. For heavy, use brass beast on this wave. Stock is fine and natascha is a good utility if the scouts are really fucking your team over, but brass beast excels the most here. For engie, use rescue ranger, wrangler and STOCK wrench (or southern hospitality, I guess). The superior building healing will be the best for tanking your sentry. Additionally, buying building canteens during the wave can be very useful. Anyone can buy upgrades during the wave, of course, but building canteens are cheap and extremely useful, whereas stuff like damage upgrades on other classes can be extremely expensive and not available until the wave is about to end anyways. First medic should ALWAYS use kritzkrieg, medics after that can use vacc or kritz. Sniper shouldn't be used, but if you really, really, insist, use heatmaker. Spy is so much more a meme than even the other bad classes for this wave that I won't even suggest a loadout.
For general strategy during the wave, I've already talked about setup, so play around the engie base and pay attention to your demo. If he dies or runs out of traps, start attacking as much as possible to pop the medics that might be left. Until then, hold off until the soldiers start rounding the corner. Eventually, the base will fall. If the base falls, at least one engie should always fall back to bomb hole. If the robots are coming down the left path and staying on the ground, then smaller bases or holds can be setup and/or made wherever they are. If they're coming on the roof, most of the team should immediately fall back to the bomb hole. The only ones who should stay following and attacking the robots on the roof path are mobile classes, most notably scout, who needs to collect credits, and classes which can either damage from around corners or can do KB. Demo is the most obvious here, although having a trap on the bomb with the Scottish Resistance as well is a highly recommended idea. Soldier can also serve well enough here, especially if you can buy rocket specialist during the wave to further slow down the heavies. Offensive cooldowns, most commonly kritz, should be used to burst down the heavies when they are available. KB can be used to delay the heavies while charge is being built. As mentioned earlier, the heavies can be used for ubersaws, and so can the shortstop scouts if one medic heals another.
I think I've mostly run out of things to say here. I've passed 15000 characters while writing this, so it's gonna need at least 6 posts, jesus christ. Tell me if you think I've missed anything significant, you think I've said something wrong or if you think there are better options.