In Chapter 1: Part 3 - The Dead Bog, kill the Swamp Troll to get 250 XP.
Then, restart from the last checkpoint, and kill the Swamp Troll
again to get another 250 XP. Repeat this as many times as desired.

In Chapter 1: Part 3 - The Dead Bog, once you reach the large goblin
camp where you have to blow up the tree, keep killing the goblins to get
easy experience. They will keep respawning until you hit the tree with
two bombs. Repeat this as many times as desired. *Note:* You can also
complete the trial for killing 50 goblins by using this trick. -From:
thomasstaggs

In Chapter 2: Part 7 - Sanctuary Entrance, kill the Stone Titan to get
1,000 XP. Then, restart from the last checkpoint, and kill the Stone
Titan again to get another 1,000 XP. Repeat this as many times as desired.

In Chapter 5: Part 2 - Wygol Village, once you get to the third ghoul
hole you need to plug up that is next to a neutral magic font (behind
the wall), ghouls will keep respawning approximately every 20 seconds as
long as the hole is not plugged. You get 50 XP for every ghoul killed.
The Holy Cross power usually kills them with two hits. Repeat this as
many times as desired.