My guys are level 7 (War), about to get to level 8. I use the basic slash offense, with level 6 activity and level 5 blood lust. Also they aim for the torso. They have level 6 blade skill. But I am not even getting a shot in on Grom. He is just destroying my guys, usually in about 30 seconds. Unless it is something obvious, I will just avoid the challenges for a while.