Raincoat

[figMiNT]

I would use the example of how if one Warrior is in an aided combat then all that's needed to win is a tie to help explain this answer. Just like how you imagine you are facing one giant Monster, imagine the Monsters are facing one two-headed, four-armed Munchkin. So no one can play potions against you [or him/her aiding you in combat]. But if someone is aiding you in combat, they can play potions normally (for or against you).