I had the idea of Dark Mirror as a more of a stalling spell than anything like an attack spell.
The 'cheapness' comes from the hand spell, which would be overpowered if it werent for the battle-rage that suddenly comes over the user of the spell.
Ex. creates a copy of Sain's character, with dolls and all, then also starts the incantation. Next post, he and the copy (if Sain didn't kill it after his post..) surround and attempt to remove Sain from the land of the living...
The battle-rage forbids anything that tactic however, and its more likely that Jelkin will take out the copy during his rampage, and hopefully Sain's character as well.