Demons

Just a quick question. Has anyone had any success using magic against Demons and Demon Lords? (other than summon demon ) Neither lightning, ice, or fire seems to do any damage so I've had to stand outside a doorway with my limited bow skills and shoot hundreds of arrows for 10+ minutes each!! Tedious and a PITA

I did the same. killing them with tons of arrows and bolts. it might sound tedious, but it somehow feels rewarding too when you kill them. of course, you can always try to catch them in a corner and constantly hit them with your sword. ( for fast enough hits, I think Your skill level should be above 30 ) , they die faster this way. but don't let them escape.
Playing as a magician has it's advantages and disadvantages in Gothic. this is one of it's disadvantages.

If this is G2 NotR, then yes, killing them slowly with arrows is probably the best way to do it. Summoning a demon yourself is, as you pointed out, the other viable option. Most spells won't do anything.

I suppose Shrink could work, but it costs a ton of mana.