In order to beast him easily, all you have to do is bring Mistral and Piros (Mistral is required anyways). Make sure you're at least at level 36. Now you don't attack Cubia head-on, you have to destroy the core. The core sends out miniature cores, and they heal, attack, and stuff like that. Destroy all the 'Repth' cubias, and then all the attacking ones. MAKE SURE YOU ONLY HAVE ONE LEFT. Next, you just beat the snot out of the core until it dies! But be warned: Do not have your Partner(s) attack the core- have Mistral heal, and your other choice(preferably Piros) Attack it until it dies.