two ways to look at it, have it be ~30% instead of 50% and allow stacking, but the %s not adding together, but rather being sequential, 30% reduction and then 30% reduction of what is left, then 30% reduction of what is left (3 cards out) so if there was a SoG out it would be 5-1-1-1=2, if it were a buffed vampire with 20atk it would be 20 - (30% of 20) 6 - (30% of 14) 4 - (30% of 10) 3 = 7
or
have the % be lower but stack directly, like 20% per stack. the SoG with 3 out would be 5-3=2, the buffed vampire would be 20-12=8
keep in mind for the vampire the damage done is not altered.