It's impossible for a hero to trap your villain in a PvP zone and keep you from leaving. Whenever you respawn at the hospital, no players can attack you for the next 30 seconds. It's not possible to be killed as you respawn by camping heroes.
I'm pretty sure you can get it as often as you want. It can be activated five times, and each time it summons a level 25 Shivan Elite Boss. pretty powerful.