You could just card everything. I was opposed to this (and actually still am), but if you want to not gain any levels, but gain AP, then carding is the way to go. When fighting, just damage the enemy until it has very little HP left without killing them, then use the card command (You can learn the card command from Quezacotl early on.) This gets you AP, but no exp. Then you can learn all your GF's abilities, without limiting yourself to just boss fights, without gaining exp, thus keeping your characters at low levels, and therefore keeping your enemies at low levels, which keeps them weak.