Best I've found so far is a Black pudding (level 7) will kill at level 14 Grikkitog who only deals damage the pudding is immune to. Now the Grikkitog can simply burrow way, and has an intelligence score to know it's losing, so instead why not the Giant Aukashungi, who can only do damage the jelly is immune to and is too stupid to run away.
So that's my favorite so far. What all can you come up with?