Level 55: 3500 xp per mob

Move to Ramparts with alt.

Bring in shadowpriest or a lock. Also bring your alt along (2 accounts obviously), both enter the instance.

Have your lock dps the mob down to a low% health. Dot the mob up and have the lock run out of the instance. Now the mob is inside dying from the lock dots applied. Once the mob dies the alt inside gets the full xp as if it killed the mob itself.

The trick is working when to run out, too late and the alt gets the penalty of having the lvl 70 in the instace, too early and the mob could pwn the alt. I like to fear the mob if possible just before I run out to give that extra bit of safety to the alt.

For the quickest xp I recommend doing the mobs at the start of the instance, then run out/reset and rinse repeat. Hope this helps some people. Have just used this tactic from 55-58. This is not my idea, but have no yet seen it here so am sharing with you guys. Hopefully can help someone who hasn't seen it yet and wants to hit 58 ASAP to start hitting up Hellfire quests.