I did a playthrough with a friend: he (orange knight) spent his whole experience in strength, while I (red knight) spent mine in magic. Most of time, magic seemed to deal more damages, but it was pretty hard to hit several enemies at once. Even if, most of the time, I killed more enemies than he did, he always gained more experience than I did (sometimes, almost twice more).
I already know that the Giraffey pet can help to level up faster (10% xp bonus), but above that, which choices can help to level up fast ? Are there any easier characters for that (with area magic skills, for instance) ? Is strength a better investment than magic ?
Regarding the red knight, I've read that the he was much appreciated for his "Lightning Hold" attack that can "splash damage", but it seemed really hard to perform: you have to be on the exact same Y axe than your targets to make it work...