Immortal aspect? (the deep)

I try to make a run into the deep with only my 7 heroes, but when i try to fight one of the first 4 aspect of Kanaxai, I reach to damage those minboss only for 1/4 of their healt... after, thei became immortal (making 0 damage).
I try many time and all the 4 aspect, but the result is the same... someone can tell me why?

Hi, you will need a knockdown skill to get past them and eventualy kill Kanaxai. When you stop doing damage do the knockdown and then you can hit them again.

Thank you 1000, I've also search some walkthrough, but no-one tell about knockdown

thanks again!

http://www.gwpvx.com/Build:Team_-_7_Hero_Deep