its a bugged happens to random mobs for instance for me one was the nercomancer in the flying forts would pretty much hit me once with a pioson arrow /spell thing take me down to 50 hp and the tick of poison would then kill me .I also had the same thing happen to me with the dragon terror patrol they would hit me instantly for 1900 damage , a few other mobs as well like i said its a random thing it take s alittle to work through it or to go back to a save piont leave area and come back a little later even if u just port to your battle tower and back that sometimes works
