I've made a brawl stage with the goal of making it impossible to be killed. I technically failed, as it is POSSIBLE but extremely unlikely. The stage is basically a gigantic box with a ceiling made of falling blocks. Once inside, it's nearly impossible to die. I've brawled for over an hour at 999% without dying. The only 2 ways dying is possible, is if an item causes a block to fall and someone gets smashed through the hole in the ceiling (this has happened once to me, its extremely unlikely). The other way is to get a warpster/dragoon etc and jump off the level.