This seems to work best if you have 2 higher levels lackeying 2 lower lvls. But we have done this with 4 lackeys and 4 masters.
We have been testing and the cut off looks like 9 lvl difference max... feel free to try other lvls. This was the best XP on lvl per lvl.
LvL 25 lackeys LvL 14+
LvL 25 grabs mission. Now, you can have as many as you want for pairs, but the ending XP will vary on the group lvl and the lackey to master lvl.
Everyone goes into mission and kills things like normal. You should be getting good XP on the normal mobs. On the final Boss \ Rescue \ etc. The lackeys must UNLACKEY before you kill the last mob or boss or rescue... whatever the mission is. It would be wise to find a place to hide while unlackey'd. Now the higher lvls just keep the last mob alive... dont kill it right away... or dont run out of the instance. Wait about 1 1/2 - 2min. Either statis the last mob or let him beat on someone while the other heals the person taking the beating. If its a rescue or escort type mission.... DONT click on the door yet... and stay back away from the exit if you are the escorter. After the time I mentioned above has expired... complete the mission. The XP will roll in.
I have been getting 1600-1800 XP per mission (ending XP that is, not including the regular mob XP) at LvL 14 lackey'd with a 25. XP will vary depending on your lvl. I understand that the mission XP is much greater at a higher LvLs. For example 35 lackeying 24.