A magic-null unique. This entity is completely resistant to all types of magical enchantment and/or harm, which is to say that neither god powers (like Scabies' leprosy or Dulcis' charm effects), taming (or harden/change material scrolls for the corpse), musical instruments, artifact weapon powers, rings, resistance/stat boosting equipment, nor even simple power enchantments apply when figuring out how badly he is hurt by a weapon or how well his equipment protects him from harm. In other words, your Vermis will not teleport him, your Ne'erc will not drain him, and your +7 whip of thievery will only be as effective as an unenchanted whip of the same material is. You *can* befriend him, but only on first meeting him and you must not have any enchanted equipment, wands, magical instruments (lamps included), or scrolls of any kind on your person (picking up and using anything in the list after that will be seen as a hostile action and he will attack you for it). This goes both ways, as a leather armor of great health +1 will not, for example, grant him a boost to his endurance nor even protect him any better than a normal suit of leather armor (not that he'd ever willingly pick such an object up), and he will also not benefit from a friendly mystic light frog as much as you will.
EDIT: Far as the dwarven grenades are concerned... the one with concentrated magic (and any clouds that magic mushies make) won't do anything to him, but he will take the full hit from skunk smell and mustard gas. Also, if it's not clear, SoCM/HM'ed items will act as if they always were the material they were changed to (for the purposes of figuring out how much damage he takes).
Also, gonna repost an older idea that got eaten by the server scramble a month or two ago: the doom siren. Her song induces all who hear it to suicidal acts, which translates to drinking acid, poison, breaking/zapping wands at yourself, and other such things which eventually degrades to beating yourself up with your current weapon (only once, but it will still likely hurt).