How did you make it all the way to disk four at a lvl 25? o.o;
Levelling up is irrelevant in FFVIII. If your party's at level 25, then Adel will only be at level 25 too. Keep it that way. It's far easier.

The best strategy for Adel...?
Have characters with high Strength in your team. Squall, Zell and Irvine are a good group. Use Cerberus to get Double and Triple status on everyone. Cast Meltdown on Adel, cast Aura on your team, and then let loose with some Limit Breaks. You might want to have Quistis on your team to support with spells like 'Mighty Guard' too.

Physical Limits are perfect against Adel, provided you use Meltdown first, to maximise their effectiveness.