If it is the FF2 I'm thinking of...Go into a battle with a really weak monster. Take turns kicking the crap out of your own party. Since stats are based on how you use them, you'll get 9999 HP in no time cause you keep hurting a certain party member, while doing that swich up the weapons. If your a level 16 in swords, but a level 3 in Axes, use axes to boost the hp. Also, use spells. You can increase your mp, and level your spells, as well.
Hope I helped. =]