Castlevania: Lords of Shadow

The fastest way that I have discovered to get experience is to go to Chapter 3 - 1. The level is called the Three Towers, and is fairly easy. Save up 1 Power Gem and don't use it. When you get all the way to the end of the level, where you have to start re-opening the gate just in time for 3 Greater Lycanthropes to come out and break the dial, the game should autosave. Now use the gem, and skip the gem cutscene and all 3 of them should die, giving you 100 experience a piece. You don't have to wait for the experience to actually appear in your bar, you just have to wait until the Lycans erupt and the numbers come out of them. Reload to the last checkpoint, figure out the earliest point to skip the cutscene, which is at Gabriel holding up the the gem, and then repeat this.

This should gain you 300 exp. about every 15 seconds if done right. It does get boring though, but it is the earliest and the fastest method that I could find...
The one other thing I should note is that sometimes, the third lycan takes about 5 seconds to climb over the edge, and seems to spawn after walking to a certain part of the area and then walking back, which could be factored into the equation. Most of the time though, if I wait about 2 seconds and then use the gem, he still climbs in and gets dead as the demon is creating the whirlwind.