1. Get a group of 2-3 good players
2. do pulls of 2-3 mobs at a time
3. Drop ults
I believe a group of 2 gives you the most XP, though not sure in 1.6. If you can't, or don't want to, do Craglorn, you could try Coldharbour. There are a couple of grind locations there, mostly fast spawning zombies, and at VR14 it's like god mode.