ToME Wiki
Shooting round corners
Everyone knows you can shoot around corners.
##### .@..# ###U# ###.#
You can shoot the demon, and it can't do anything to you until it moves around the corner. The automatic targetting will help you out here. But you can shoot around further corners if you do your own targetting.
########### .@........# #########U#____* #########.#
If you target the asterisk (regardless of whether it is in LOS), your arrow or bolt will hit the demon. The method for finding the asterisk: move n squares right until you are left-and-above the U, then move the cursor to the U, then move n-1 squares further right.
You can expand this to get multiple free shots on non-tunnelling creatures. Let's say we have this setup:
###########################.# ###########################U# ############################# ...@######################### #############################
Target the square directly below the demon with a dig spell. You will dig out as follows:
###########################.# ###########################U# ################...........*# ...@............############# #############################
The demon will start moving towards you as soon as you open the square diagonally to its left. The square below the U, which I have marked with a *, is the only point from which it can target you. (you need not dig out this square) But if you keep that square targetted, you can hit the U with bolt spells with impunity until it turns the corner and enters direct line-of-sight.
You could even employ stone prison and home-made void jump gates to bounce back and forth. But if you can't kill a major demon in twelve shots, maybe you're out of your depth anyway 
(obviously tunnelling creatures present a bigger problem)
Comments
I figured this out all on my own. Apologies if it's already known to the community. Is this an abusive tactic? I'm finding it quite useful..
NeilStevens: I don't consider it abusive. This is how ToME 2 LoS just *works*. Sometimes it works in your favor, sometimes it works against you. We're making LoS symmetric in T3 though, heh.