I think a perfect counter for that is mass spellbreakers they would jsut take all the bloodlust LOL. But when you mass casters its really funny when you jsut select all the witch doctors and they all put a healin wards down its like kool cuase you hardly lose health
but i never mass caster anymore unlees you ahve a partner thats massing a good melee like tauren or aboms or else they get wasted.