Best 5th-level spell.

In battle - explosion (though it won't help with anti-magic - summoning earth elementals is better), on the map - Dimension Door.
The mirror, as I recall, has a 30% trigger rate, meaning out of 3 hits it will reflect 1 and let 2 pass through. And if it lets the first 2 explosions pass, there will be no one left to reflect the 3rd...