try using Devotion aura(copy it) and than shift double click, enter amount you want for each lvl(like for 5 arm reduction use -5)(you can add custom values depending on some events by triggers but you need more levels) and than select aoe, select targets to enemy, ground, fly, ancient, w/e you want but if you don't want your units to be affected uncheck friend, self so it won't affect your units and the hero