My favorite class WAS spy (tied with scout), then I learned how to airblast with the pyro and now I have a new best friend in MvM. I mean, ok, this is nuts, I just completed the first wave of medium difficulty Decoy with only myself, another pyro, and an engineer, all because I can just keep blast blast blasting the bomb off the bridge into the gulch forcing them to spend a significant amount of time pathing out of it, and about half the time they path the longer, more time consuming route.
That, and I can protect my engies from sentries by keeping the Sentry busters suspended in midair unable to move, and cheese the giants with medics by blasting those medics away. Maxing out airblasting is a must now, it can't be ignored, and that's easy when it's so cheap.
I'm a newer player, so of course this is old news to more experienced players, but to me this is astounding because I'd only ever see the occasional pyro blasting the bomb carrier to reset their route, and that made me think it was some high level technique that's not used often. From my experimentation though, it's the opposite, you can troll the robots by repeatedly forcing them to redo their route over and over and over again. I did it six times in a wave, it was awesome. And when I'm not blasting them to reset the bomb, I'm blasting the bomb carrier and entire groups of them backwards to earlier in the level; I once pushed the bomb from the hatch all the way back to the enemy spawn. Take that!
Instant new favorite. I just wish that airblasting worked better on giant bots rather than just bounce them in the air for a second, which doesn't even faze them.
I even had fun with some random guys, and we formed a four-man pyro patrol, and with two engies we steamrolled over that mission.
Now I'm starting to wish there were giant campaign missions you could participate in with many more waves, cause now the medium difficulty missions are too easy for me, but the advanced difficulty ones are still horrendously difficult, and I want the difficulty to keep ramping on the medium missions so I can keep putting my upgraded stuff to good use rather than having to start over again. I'd even like to keep going so long that I might eventually max out every stat. Some sort of infinite mode would be awesome, that starts off simple but keeps getting harder and harder would be awesome.