WTF, some guy is H4X0R-ing me. He keeps changing marks, and healing himself to 200 health each turn!
Chances are, this person isn't hacking. It's a bug known as desync, and it can cause all kinds of strange things to happen, including mark changing and full health healing. Your best bet is to re-do the game. Of course, there is a small chance the person you're facing IS a hacker, in which case, you should report it by sending a PM to the Administrator of the forums, Scaredgirl.