Saw this issue yesterday. Somehow enemy was pushed into one of my my characters, they occupied same space, graphicaly overlaping, both were stuck couldn't move nor attack.
My character was on top it seemed, cause I could not attack the enemy even with other toons. I solved the issue by attacking my own character killing him and then killing the stuck enemy.

Maybe not exactly related but when creating a full party of custom mercenaries, two of them spawned and got stuck in terrain near tikiwara marketplace. I fixed it by removing them from me party, moving a bit to a side and adding them again.