How can you change the damage delt when a unit is damaged?

Is there a way to just subtract x life from a unit? Right now it seams the only way is to set the units life to itself - what I want.

set (attacked unit's life) = (attacked unit's life) - (x + 30)

Thats the only way.

or..
if you want it experience and bounty save, create an invisible unit which casts a damage spell which does that damage to your unit