I've always just detected his flesh and then used fireballs.

My best guess is it's a reference to the film Magnolia, in which it rains frogs. It's not a film I would recommend.

I
Before going for the shirt I tried to assassinate Petrus with just explosives - tamed all of the cathedral dwarves, stacked their backpacks on one of them, along with a few wands of lightning, fireballs, striking, and the only wand of acid rain in the game. When I aggravated Zak the kamikaze dwarf of Valpurus, he died partway through the explosion, and dropped all the unexploded items on the floor. They were whisked away to the dwarf room, partly intact - the wand of acid rain broke.

Oh, veterans have 3 int. They're still undetectable by ESP, though.

Especially with strong doors like the mage room's.