Another Mega Rayquaza
I don't do much AG, but I should think that Primal Kyogre would be a good answer. Switch it in after M-Ray has Mega Evolved. It will then undergo Primal Reversion, setting up Primordial Sea and removing Delta Stream. You are now bulky enough to take a Dragon Ascent or Outrage and fire off 4x supereffective Ice Beams.