I attempted to activate Polukranos, World Eater for X=0 to be able to target any number of creatures for 0 damage and it would not allow me to choose targets. I was attempting to target a copy of a creature created by a Phantasmal Image to kill the creature with the target of the ability, even though x would have been 0, but the game would not allow me to choose any targets when I should be able to.