just make it so that the defibrilator changes the death system so that if a player dies, their body does not actually respawn until they actively click to respawn. while their body is lying on the ground if someone uses the defibrilator close enough to them they come back to life on the spot where their body died with no consequence at full health (the idea being that if an area was dangerous to kill the player in the first place, if you revive them with less then full health they'll just get shot on the spot again)
in slayer, if you die on your last life, your corpse will stick around till the minigame restarts but you can still spectate n stuff outside of that
in order to actually 'use' the defibrilator you have to click and hold the button for a couple seconds to build a charge. if you're near any corpses when you release the charge, they get revived. the idea behind the charge mechanic is that a defibrilating player becomes a target, but not a totally helpless one; he can still dodge fire, and has to hold out long enough to get his friend off the ground without returning fire or else he'll loose his progress
