Just thought I'd mention that based on in-game experience(less than ideal, but it's mostly what I have), my Cresselia(non-EV trained(some EV's but not in any strategic way), modest nature, average IVs; I'm fairly new to competitive battlinig, so I didn't know what EV training was when I captured her) completely walled my friend's Mega-Alakazam(EV trained in Sp.Atk and Speed, Timid nature, average IVs). Cresselia would have been barely 3HKO'd by supereffective shadow ball, and had moonlight and dual screens to further increase its walling ability. Cresselia's 4th move was Ice Beam, which hit decently hard on MegaZam, so I think that it is safe to say that Cresselia could sort of serve as a check to this Mega. Granted, Cresselia walls virtually everything not named Mega Mewtwo or Arceus(there's probably a couple more, but idk right now), but I believe that this needed a mention. Cresselia could also Toxic the Mega once it loses Magic Guard and watch it slowly die to residual damage.