I did it by leveling Bush whacker really high level, then go back to level one, wait for twenty mobs to be on the screen, and kill them. You should also turn off auto progression, but you do need to progress after you kill the 20 mobs.

Thanks guys! Gonna try one then the other if I don't get result.

I built up my guys to about stage 90 like I normally would, except that I didn't buy the final abilities. Those are the ones that massively increase DPS for every 25 levels at 200 and above. I bought lots of levels for Jack and the Hermit.

Then I switched to a new formation. Jack and the Hermit go on the front row. Since they're very high level, they have tons of health. Khouri goes on the second row because he has healing, and the Gryphon goes on the second row too, since he reduces damage. That's it, just four crusaders. This has massive health, but virtually no DPS.

I went back a few stagesand found a good place where the enemies can't do more damage than Khouri can heal, but my DPS wasn't strong enough to hurt them. Maybe stage 75. I counted to 20 enemies, then bought the Hermit's Insanity ability. Instantly he had massive DPS that killed everything. I just had to go to the next stage to get the award.