I don't know if anyone posted this smart leveling trick yet, but I'll post it anyway.
Let me start off by saying, to get 100% use of this trick you should have a character with AoE who is around level 54 or above. And furthermore, a friend whom you'd like to boost.
Here's the trick:
You may or may not know it, but there's a boss in the Eiglophian Mountains named Diabolist Aino (Coords: 908 , 768 - aproximately ). He regularily spawns 4 level 54 adds (not minions, meaning they give good exp) which are pretty easy to kill. So, say you got a friend you'd like to boost some levels, gather up outside the camp where the boss is in, put aprentice on your friend (for survivability, though I've experienced more experience gain if not in aprentice mode), clear the camp before pulling the boss. When you pull the boss, be sure not to kill him, just let him use all his mana, when he's dry all he will do is teleport around and adds will regularily spawn. So the trick is to keep the boss alive while you just aoe the 4 adds (and optionally the eventual spawns in the camp), this is like AoE grinding level 54 mobs, just with a very fast respawn timer Wink.
I did this with my friend (level 35) he got 5 levels rather quickly, I didn't get that much myself (level 56, tempest of set) though.