First off, if you're using Selphie, that's a big no-no. Selphie's just about useless.

Before the battle, focus Squall's junctions into speed and strength. Use Zell and Irvine (Whose Limits can hit only one target.) as long as they're high in level; otherwise, use Quistis as long as she knows Mighty Guard and White Wind. Now, in the battle, cast Meltdown on Adel. Then cast Haste on Squall. And, if you junctioned him right, he'll deal massive damage very quickly. Have the others use limit breaks to support Squall. I'll bet that'd do the trick.

If you really need to level, as long as you have the Tonberry GF, you can raise the levels of the monsters you fight in battle with Lv UP command; all monsters in the Lunatic Pandora are at level 1 before you mess around with them in battle.