The more HP you have, the more gold you get when beating Chara. So healing to 99 HP before you win would be the best method.
For no-hit, just practice the attacks, learn how they work, and develop a strategy for each one. Most are fairly easy but some can give some trouble. You can also use the Gelly Fish from the shop, which gives you a shield which absorbs one hit, and makes that hit not count towards losing no-hit. I also recommend using acts instead of cutting her clothes, as it takes less turns.