If you are a class that supports high damage over distance, you can use this guide/trick to take out some higher level mobs pretty easily, for great XP.
1. Go to the Acheronian Ruins and work your way up the steps toward the Bat Demon.
2. After the fourth set of long stairs there is normally a large spawn of Picts around level 17-19.
3. After the stairs, when you come up on the platform, there is a ledge to the right.
4. If you jump down on the ledge before or after the Picts see you, they will stand around trying to get you, but they can't.
5. I would go to the end of the ledge, where the spawn is normally pretty quick, and just start nuking.
The Picts will not venture down on the ledge and I haven't encountered one that will hit me. They stand there, fluctuating between brandishing their weapons and walking around. So, you can nuke/shoot them quite easily. There also seems to be a pretty steady flow, so happy hunting.
~~Also, I've found this to be a common thing...the system isn't handling line of site and barriers well. You can also stand on stairs and shoot/nuke and the mob will get stuck at the top of the stairs. Some of it is trial and error though.