Alright, first off, low levels are unacceptable with this boss. I was at level 36 when I beat him on critical, so you'll need to be at least level 33-35.
Here are abilities you HAVE TO HAVE to beat this boss. The recipe for making this ability is shown in parenthesis:
Once More (Fira+Fira+Wellspring Crystal)
Last Chance (Blizzara+Blizzara+pulsing crystal)
Scan (if you don't have this, facepalm)
Here are abilities that help alot, but aren't necessary:
Magic Haste (cure+cure+fleeting crystal OR Fira+Fire Strike+fleeting crystal)
HP Boost (Stun Edge+Block+Soothing Crystal)
Here are Commands that you'll NEED:
Renewal Block (Curaga+Block)
- counter hammer
Curaga x2 or x3 (buy it or cura+cura)
These commands are very helpful, but again you won't need them.
Fire Surge (Fira+Fire Strike)
Ars Solum (Dark Haze+Stopga) NOTE: This command only has a 20% chance of being made when you meld Stopga and Dark Haze. Before you meld them, save your game. If you don't get Ars Solum, turn off your game and try again.
For example, here's the deck I was using:
Ars Solum
Fire Surge
Fire Surge
Fire Surge
Curaga
Curaga
Curaga
Renewal Block
- Counter Hammer
Slide
Sonic Impact
Air Slide
High Jump
and my abilities were:
HP Boost (maxed out)
Magic Haste (maxed out)
Second Chance
Once More
Fire Surge and Ars Solum are solely for defeating MX and Vanitas. When you're facing Terranort, don't try to attack him AT ALL. Only block his attacks and deflect them back at him. When he shoots those black orbs at you, DON'T run away. Stand your ground and deflect them back at him. Due to your Renewal Block, not only will you be healed by deflecting these, you'll also hurt him by deflecting the attacks. When he attacks you with his heyblade, block it and fire back with Counter Hammer. Those are the only two ways you'll be able to damage this boss at all if you want to be safe. When he slides at you, you can block it to get some free health regenerated. It's near impossible to completely dodge or block the meteors, so get as much distance between you and Terranort as possible and let them hit you. Once More will let you live, and since Terranort is so far away from you just use curaga to heal off all the damage that was done to you.