Whats wrong with Namor in Zombies?

So, i'm able to clear each wave in less than a minute, but everytime i get to namor and wanda, i'm stuck for 7-10 minutes. Its horrible.

So, how do you get namor actually down? I realized the big squids sometimes have blue, red or green rings about them, and sometimes they dont do damage and sometimes they even heal him... is there some trick thats not explained properly?