Lets see... Aco and Priest leveling is highly dependant on your gears, and cast time.
But I'll do the best I can;
Level 30~whenever your gears allow you to stop:
Prisons, Churchyard, Amatsu dungeon (Healing Firelocks) & other places with large populations of undead monsters.
Possibly level 60 or 70+ :
Ancient mummies (Morroc Pyramids) It again, depends on how well you can survive. If you need help - try to find a Crusader, knight, or Steelbody Monk around
your level to party with. They're usually the best tanks, and it would allow you both to level fairly quickly.
And then depending on your gearset, you should be alright to attempt Nifl.
At first stay behind tree's, or small spaces, out of the way from other people. Less monsters will mob you, and you should be able
to proiritize surviving - over picking up your loots, before a Blacksmith greeds you.
Just a rough guide, but it has some leveling tips I guess. I never was really good with the priest class.
I suggest you post in the Priest/Highpriest topic if you want a more indepth guide type thing.