I'm thinking dispel/curse/blind/slow/plague and maybe weaken would do the trick if you got a decent fiend. Though, maybe I'm underestimating Kornuel.

I'm not sure what weapon he wielded, but figure plague + weaken + your bard song is going to put him at around -20 even if it's a crappy weaken. If that makes him drop weapon, and he's slowed, and blind so he can't wield a healing staff or eat pills...could be rough.

I just know that if I were in Kornuel's shoes and you got me slept, and had all the time in the world to sing/recite/brandish at me, then I would be somewhat worried.