Priest on Medusa level
Finger-of-death instakill by an Arch-Lich

On level two, I ran into a roomful of monsters who were all lined up
nicely.  I had a wand that I had not yet id'ed, and didn't think I
would make it through the confrontation, so thinking what the heck,
zapped them.

Wand of Polymorph ... and I made some of them bigger and badder.
Crap.  Oh well, may as well zap the pet and see what I get.  Poot!
Jackpot!  A Jabberwock!

All the rest of the game, I just stood back and let my buddy rip his
way through monsters, shopkeepers, whatever, while I picked up the
leftovers.

I took him through the mines, sokoban, and a ways down in the dungeon.
At one point, he bought it, and I was pretty sad.  Shortly thereafter,
I acquired another dog as a pet and had one charge left.  Poot!
Another Jabberwock!

Much fun all the way down to the Medusa level.

The general consensus for your first scroll of genocide is to wipe out
all of the Liches (L).  Consequently I have not had much experience
with them.  Earlier in the game I killed a regular lich at range.

When I hit Medusa, I ran into an Arch-Lich that kept creating monsters
all around me, which I was dealing with, but then moved to the space
next to me.  I got one hit in, then he finger-of-death instakilled me
before I could do anything else.  A week of gameplay gone.  After I
finished my howl of frustration, my next thought was "Hmmm, I didn't
know they could do *that*"