Deathsquad of Immortal Uruks. They should really fix it that Uruks don't get unkillable like that.

I got the first one to burn, but when I attacked him, he still blocked everything and just because he was terrified and green, he did run away, but I couldn't kill him.

The 2nd one is afraid of Caragors, but onehits them and they die instantly while he's fleeing still.

Both of them actually managed to kill my Graug while I was sitting on it, too - they're simply immortal.