if he was continually casting he must have been using god mode or a mod that allowed fast regen. i think you can get a max of three or four of those spells off before you run out of magicka. as for the AI running to the same spot, ive done that tons of times with bad guys in oblivion. i would get raiders or undead to walk into the spike traps or the moving walls of spikes or the swinging blades and make the traps do the work for me.

i think its just a universal part of gaming that your cannon fodder enemies are total knobs.