When Toya asks you something at the end, you will have to choices. You want to pick the second choice which is "Never!". Then proceed to destroy the last bosses.
Now read here:
Get Omega. Use the Counter Evade skill, and go to the bottom of the Holy Beast Shrine. If you have Yuki, and her Sniper Shot. Omega will be a breeze to beat.
Then level him up to like lv 18, which would be like 3-4 fights at the bottom of the shrine. Then make sure that his health is above 1000, and his EP is above 760. This shouldn't be to hard.
Have him in your party when you face Infinity(after the queen of ice). He should be able to use "Near Death Edge" on all of Infintys forms, effectively single handedly beating Infinity himself. (I have not tested this on the queen of ice). After everytime he uses this attack, make sure you use a Gods Ambrosia, or Gods Liquid.
This is an easier more effective way to beat the last bosses, than taking the extra hours to lv up.