I know that the max lv is suppose to be lv 99, or at least i assume that. but i was wondering, is there a limit to gaining AP in battle other then if your character has every technique?

There's no set limit that I know of, no. Yes, of course, once all of your characters have learned all abilities, you're not going to get anymore.

There is no limit to how much AP you can get in a battle, so you can constantly use abilities to obtain AP throughout the entire battle. However once you mastered a dress sphere, you won't get any more AP for that dress sphere. Or maybe you do get more AP, but you have nothing to use it for so it goes to waste anyhow.