[0.51] Mana Shield/Blast
In games with a slow pace, mana reserves can build up to substantial levels. Mana Shield (which allows you to absorb damage with your mana stockpile) therefore becomes an invincibility spell, while Mana Blast (which deals 10% of your mana as damage) is an almost guaranteed 'I win' button. I tried casting Mana Blast at a dragon. It resisted and took 474 damage, and died.