For Terra the best commands for the final battle i reckon:
1. Fire Surge or Thunder Surge, I prefer Thunder Surge since it has a wider range to hit the enemy. Also, when you use any of the surges, you become invincible while you are dashing, like a normal dash. Have like 2 or 3 of any of these
2. Ars Solum is great once you have guarded their attacks and they are open (Renewal Guard is great since you also recover HP).
3. Curagas. Like 3 of them. You will need to heal if you are caught in a long combo. That said you would probably need the Once More and Second Chance abilities.
4. A darkness themed deck is stupid since you won't get a chance to enter the Dark Impulse command style, since the chances to hit the final bosses arn't very often, and if you do try to hit them very often you will find yourself dead.
5. If you really want to use Dark Impulse get the ice cream that shifts you into Dark Impulse, i'm not sure which one it is, but it does exist.