Weakness to Magic stacks exponentially with itself when it comes from 2 different spells.

For example, lets say you have 2 spells with Weakness to Magic 100% for 6 seconds: Spell 1, and Spell 2.

Spell 1: target becomes 100% weak to magic
Spell 2: target becomes 2100% = 200% weak to magic recast Spell 1: target becomes 2200% = 400% weak to magic recast Spell 2: target becomes 2*400% = 800% weak to magic

And so on. After a few stacks of that debuff, and you can 1-hit-kill any foe with a spell that deals damage.

Weakness to Magic also works on the player character and it will amplify the effect of buffs. Stacking it before casting your buff spells lets you walk around with millions of health, stamina, and mana for short periods of time.