And as a final image, this is what the game looked like in the last wave. It was chaos, frames were down because everything shot like 20 projectiles at once. 
A final addition two days later: I was able to do it again with pretty much the same setup, on the same map. Range, burn, ice, poison, stun, fear and damage amplifications seem to all play a key role. Especially the 10% damage amplifications when stacked seemed to do wonders. The enemies slow to a crawl with multiple slowing effects all stacked, and on top of that, high stun and fear chances etc.
Highest damage dealers were as follows:
Bashful's damage +24000 M, Sleepy's at +16 000 M, Ice at +7600 M, Poison at +6000 M and then at fifth, Stormivan at <2000M.
I think that Bashful and Sleepy are both incredibly powerful even on their own right, but it's also that they both were in range of the first straight on the map, during which all the fear and stun effects procced, so they easily had much more time to deal damage than most of the others. But even so, the difference between Bashful's damage and that of the third actual character was more than 10x.