i had this problem too with a few mobs on Tat, for me the groups just kept losing aggro and resetting over and over for no reason, i had time for maybe one skill, then they lost aggro and walked back to their spawn location and healed to full, they never attacked back and i could never kill them. I tried blowing them up and pushing them away from their spawn but no matter what i did they just healed to full and walked back, it was surprisingly common to have happen but not really a big deal.