Get out the little blue pills defenses, because I suggest a new spell- Impotence!
Spell has 2 effects- weakens all building hitpoints within its radius and reduces the damage done by defensive units / towers in its radius. Percent of 'weakening' would increase with the level of the spell
Would make an excellent intro to kick off an attack and would fit well in combination with other spells (drop Impotence, followed by lightning or rage, etc).

