If he activates ectoplasmer and you don't have de spell you are screwed. And even if you do destroy it he will just activate another one. You pretty much need de spell in your hand at all times. There's also the fact that you might have to take damage to even get Cerberus in your hand. With only 1 heal spell and all the ectoplasmers/burning land/poison You will die to effect damage pretty easily.