I might have to agree with the sader suggestion...which is funny since priest in my favorite class.
But, if you choose priest, what I found was the -best- way to level up, is to have a sader buddy. That way they don't waste so much sp on healing, and you can share exp and support each other. It's about the best team effort on ro, I think. But for going alone? Yeah, saders have more chance of survival. =3