OK so let's list the ways you can beat CM versions of Reuniclus :
-Strong STAB Super Effective Physical attacks. Megahorns (Heracross/Escavalier), Crunch (Tyranitar), Bug Bite or U-Turn (Scizor)...
-Tricking a Choice Item (Gengar, Latios, Starmie, Metagross...)
-STAB or Banded Explosions...