Complete the game to unlock Fishing mode, which allows you to fish under timed conditions and unlock pictures in Gallery mode. Also, complete the game under the hardest difficulty setting to start a new game session with all Dragon Genes.
Make sure you have Ryu at level 40+, Rei at level 30 to 40+, and Peco at level 30+. Peco must also have Spr Combo. Use Defense formation or you will die easily. Begin by having Rei transform into a Weretiger and Ryu influencing him. Peco should then cast Focus to boost his attack power. In the second round, Rei will attack automatically and Ryu will Transform into Infinity + Trance + Radiance Kaiser. Peco should use Super Combo if nobody requires healing. Have Ryu use Shadowwalk, or Aura if you do not have it. Simply use Bonebreak and continue with normal attacks. Peco should use a Wisdom Fruit if Ryu's AP is below 50. Have Peco keep healing Ryu's AP until your stock of Wisdom Fruits and seeds run out, then have him heal or use Super Combo. When Myria starts using Holocaust, have Peco use Moon Tears. When Ryu transforms back, have him cast influence on Rei before attacking or he will hurt your party members. By the time Ryu transforms back, she should lose about 15,000 to over 18,000 HP.