I highly recommend maxing out Fireball (from bone pillars) and Splash Needle (from merman) ASAP. They're both found on chapter 1, and you can absorb Splash Needle 4 times very quickly near the start of the level if you jump into the "pool" with 4 Mermen before you use a switch to fill it with water.
Fireball has the lowest MP cost of all her spells, and is sometimes more MP efficient than other spells since a fair amount of monsters are weak to fire. Splash Needle is fast, fires 4 shots, and travels until it hits a wall so it's really easy to spam it down long hallways like the bottom of chapter 3 and 4.
I would also recommend maxing Heal (Chapter 4 witch) and Acidic Bubbles (Chapter 4 gargoyle). Acidic Bubbles fires ~7-8 shots in a cluster that have a decent travel speed and rack up a ton of damage, good spell for bosses. Heal can be kind of a pain to max but it's worth it. There's a technique you can use to absorb it a bit more easily.
http://www.youtube.com/watch?v=EM6bI4bxRs4
I don't consider any other spells super important, but Energy Flow (Chapter 4 witch, same ones for Heal) and Summon Gunman (Chapter 5 skeleton) have some promise. Otherwise, just try to absorb spells when you can, since every spell will increase your melee attack power and increase the damage of all your other spells. Example: Heal started off around ~30 HP at level 1, 39 at level 9 and now heals for 60+ with some INT gear and almost all spells maxed out.
