In the original write up you pretty much become a suped up npc for minutes a day. I think the same thing could be done here for two reasons. The first being that the really crazy stuff is in the npc section. And two, ease and breadth of design for a limited use ability.
I think a low hp high skill class would be good for this purpose. If you never need to suffer the pain of battle then youd probably have a lower stamina and pain threshold. Just a thought