So against all odds a random pub team and I somehow pulled it off.
Here's what worked for us.
Wave 4: Heavy spamming uber canteens on Major Crits while firing away pointblank, while I kept rapid firing Buff Banners near the Engineer and Pyro, and we had a Medic kritzing the heavy as often as possible. This one is actually harder than the other two arguably.
5: The evil part here is bonk-drinking giant scouts that can deploy the bomb. Key here: Airblast like a motherfucker. Punt the fucker as far as possible. The rest was way easier to deal with.
6: Starts with a giant heal-on-kill heavy. It has a stupidly large health count and restores like 10% on kills or building destructions. What we did- two heavies with crit canteens being ubered and shielded by medics, while I did the banner thing with the Engi and fired off crocket canteens. Once he went down it was... okay not "easy" but manageable.
For the last three waves we just bought back in as quick as we could. Money is way less important than having six on the field.