I beat him at level 28 on Critical Mode. If you want the absolute basic tactics, all you'll need is:
- long distance magic (firaga, blizzaga, etc)
- mine shield (helps immensely, especially for his second form at the end of the game)
- glide and guard (I used it to escape his underground attacks, guard is for bouncing his combo's back and countering)
- also aerial slam is a great move for this fight, and so are moves like blizzard edge and stun edge (both have a chance of inducing freeze/paralysis respectively). Once he finishes a move he typically leaves himself open for a brief moment, so hit him with those and follow with another combo, aerial slam or a spell.
If you do that for the entire battle, it's pretty easy once you get the hang of it. Then his second form is even easier. lol I did have a tiny hp bar and rather silly basic commands. Forgive the shallowness of my suggestions. lol