>Magic Guard prevents indirect damage, such as from Hail, Toxic, Curse, or Leech Seed. It does not prevent status problems, but rather protects from damaging effects. Damage caused upon being switched in, such as Stealth Rock, and Recoil is prevented except from using Struggle, and HP loss will still occur when using Belly Drum or Substitute.
So yes, Reuniclus will lose HP. Sorry, but smart thought.
Magic Guard on Bulbapedia