I just did this on my DK/4 ret team tonight. If you completed before with melee, you shouldn't have many issues. The key is to have each of your guys dpsing at the max range of melee and they won't eat the knockback.

What I do is put my team in a pentagram all the way around him before the fight, then hit IWT once and spam dps key. This makes them face about every direction so that the mario mini bosses won't eat you up.