So I don't particularly care for Rosa all that much. She is fairly weak and her spells are just ok in my opinion. However, I restarted (I missed a lot already!) and have her in my party for Wendie's first master. With the Piki Piki Rod she is almost unstoppable! Wow!!!! She can load a TON of status effects on the enemies and sometimes wipe them out right away by inflicting exhaustion. This is going to make the Chrystal Caves MUCH easier.
In short, I stand corrected. I will now work on leveling up this previously underrated (IMO) character.