Still not sure what you're saying. It sounds like you need a full script for that, not an event command or script call.
Yeah, immortal is kinda hidden to begin with in RM (having to right click a troop enemy to access it). Fortunately, Game_Battler's @immortal instance variable is "accessible", so you can access and modify it from anywhere. A lot of classes have these that, as mentioned before, are too much to list.