Getting to 18 as a priest is possibly the easiest of all the classes. You'll be given a vast array of medium and close range grabs and melee attacks as well as the ability to boost your damage and even heal yourself. Priests can solo on group difficulties quite easily, but you'll still want to focus on a few skills to help you on your way.
As a fledgling priest, you'll want to focus on Slow Heal, Giant Weapon Launcher, and Smasher. The other skills are all very optional as these three skills alone should enable you to take on anything the game can throw at you until your class change.
Slow Heal is a rather obviously named skill that heals you or your friends over time. Slowly. It can save you plenty of those valuable health-restoring consumables, so use it often.
Giant Weapon Launcher sounds like some kind of Gunner skill, but it actually whips your weapon in front of you to your back, hitting all the targets in front of you and the ones in back of you. A great way to end a combo.
Smasher is a incredible grab that allows you to grab hold of an enemy and run him into his friends, inflicting heavy damage on the target and residual damage on anyone you happen to fling him at. Most of the early game bosses can be one or two shot with this skill with a few points in it, it hits incredibly hard.

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