Before i asked the question i did something like that, but all it did was drain all of the attacking units mana not -1, but ill try urs maybe its different
EDIT: i figured out my problem... I accidentaly put the set mana of attacking unit to mana of triggering unit - 1. So everything is ok now
EDIT again: Is it possible to make that if the unit is out of mana it cannot attack anymore? and when he restores mana h can attack?