I would have the person restrained so that they couldn't move. Next I'd cut out their tounge, making them lose their last means of communication, and preventing them from biting it off and killing themselves. Then I would place a pair of headphones on their head, playing a loud, fluxuating, screeching noise, and never take that off of them. After that I would strip them naked, and with a scapel, make as many cuts as I can on the surface of their skin, not too deep, just enough to cut them, but not deep enough to make them bleed to death. I would then take the roughest sandpaper that I could find and give them a good sanding so that their skin was as raw as I could get it. I'd then take boiling salt water(which happens to boil at a higher temperature than regular water) and dowse their body in it, repeating this process over and over(the cutting and sanding also) until they died.

Nah, straight kills for eating - this would be for fun/revenge.

Just throwin' it out there, but sugar syrup is something you probably wanna look into...

What does sugar syrup feel like in a cut? Or do you just mean scolding hot sugar syrup?

Give them five hits of acid, chop off their arms and legs, spray them with babboon pheromones and toss 'em in the cage.

Might not have the initial sting of salt, but it can pretty much keep getting hotter on it's own after a point, and won't cool down anytime quick. maybe seal in some caustic soda with it? that should cover both factors...or hell, finely ground black pepper, and salt, chilli powder, covered in boiling sugar syrup, there's a nice recipe there.