A weapon suggestion :) (COMMENT NOW :)

[GooglyBoogly]

percentage mana dran could not be made cheap, as the 'damage' which would be done by the item would increase the higher the enemy tank is or at least the higher max mana the enemy would have

I don't understand. Normally spells scale with their level, so a high level tank has more mana, and the ability costs more, this makes the cost of the spell approximately a constant %, so if you want the mana drain to have a constant effect (i.e can cast one less spell) then you need to make it scale with capacity.

Otherwise, such an item would either be too strong early game, or completely useless late game.