If you have Aura (the only person you can basically draw that from is Seifer at the end of Disc 3 and I recommend that all characters just spend the entire fight quickly drawing as many as you possibly can!) try casting it on Squall and let him keep using his Limit Break. There is the usual technique of having Squall get weak (his hit points are yellow colored) and use his Limit Breaks and if he gets killed, revive him with a phoenix down or Life spell (not with full energy) and keep the rest of your party healthy. If you have a Holy War, first let Ultima Weapon attack and if your party survives and Squall is weak and can use Limit Breaks, immediately use the Holy War after his attack and your party will be unharmed for a short time. Just keep using his Limit Breaks and pulverize him! Have your other characters use any good GF's or Limit Breaks if they can use them. Make sure you draw the GF Eden from him before you kill him also.

Hope this helps though I'm sure there will be much better tips coming along. I haven't played the game in 5 years.