Can't Rank Up

Stuck at lv.20. Can only challenge. My team is also all lv.20. Not sure how to configure combat personnel. Priests have many MP skills, but often don't have enough MP. Have fire scroll. Undead can only be dealt with by magic, but MP is insufficient. Experimenting and playing around on my own. Actually, this game is quite fun. I always want to challenge those who beat me down. The graphics are very detailed, each character and monster is.