How will I level up as a Priest?
Simple. Go into Herb Town PQ, Zakum PQ, or simply be a Support Cleric and party with DKs on Mino. Supportive and useful Spells like "Summon Dragon" and "Doom" and "Dispel" will make a STR-Priest quite popular in Parties, b/c since you do not need to raise any offensive spells. From early on, you will be welcome in more advanced parties like Zak.