You're best bet, and fastest way to level is to power through all the quests. Grinding on mobs can be decent xp, but when you can accomplish 5-10 quests an hour the quest xp is always better. The quest xp is not affected by helped recieved or who you're grouped with. With that in mind you're mage could be put on follow and priest could just annihilate everything, and only using your mage to pickup quest items and the such.

There are a lot of guides out there for powerleveling, some say about 4 days to hit 60. Most of these guides have quests and locations layed out for you. Most of them concentrate on getting quests done, and only have you grinding if you're not at a certain lvl before moving on. This could be the case with your combo as the mobs that would give decent xp solo, won't while grouped with high level. To offset that, you are able to do all the quests they say to skip because they're to hard to solo. you don't really need a guide to get the most out of it. Although some good ones make it so you don't have to lookup locations as they put them right in there for you, and even list in order what to do quests. Tell you which ones, and what you're looking for in an area. What I would do with lacking a guide is just get every quest you could for an area, then run around wreaking havoc.

Good luck