Zilgadis inflicts 2700-3500 magic damage with its mass attack. 5% of that damage is 135 minimum. It is reduced/increased by atk/def ratio between zilgadis' stats and the unit he's attacking. In essence, even a black dragon will have tough time resisting a lot of mass attacks.
I agree that with those high damage values 3% is a lot (about 80 damage by my calculations), but still, it's all about the base damage of the attack and the fact that it varies between 2700 and 3500.
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