Zalmarath the Colossus doesn't take damage

My friend and I were fighting Zalmarath the Colossus and he would just randomly not take any damage from either of us at some point during the fight. We tried it 4-5 times and each time at different points during the fight he would just stop taking damage. Also, the 1 hit kill knife hand he does that you can't really judge where he's going with it is ridiculous.