I am playing on extreme and fairly well equipped. I am in Ascended Forest Hunt, at an infected spawn. The guy with the double shields is battering my injured recruit ignoring me, and no matter how I shoot/slice/chop, he is not taking damage. Between him and the 2 snipers who finally went down, I expended over 200 shots between fire & poison rifle rounds. And I still don't know why they finally went down. I have logged out/in and no change in the current situation.
You have the Hunt Ascended minor card played, which is bugged. Enemies take no damage at all while it's active.