All of mine were at 100. My main party was Squall, Zell, and Irvine. I occasionaly switch Irvine and Selphie for variety.

BlackWaltz, your level at the end is irrelevant, really. All that matters is that you have good Junctions. I've heard of people that fought Ultimecia AND Omega with their initial levels. If you have weak Junctions, then leveling up is just going to make it harder.

There is a way to gain AP and no EXP after battles. All you have to do is depleat an enemy's HP by at least half, and then use the Card command to turn them into a card. You'll get the normal amount of AP, but no EXP.