his is a full proof method for gaining XP at an extreme ease.
You'll need a level 40 friend, preferrably a stalker, with a stealth ability. When I did this we had a 10 person group but you could do this with only you and your friend I guess.
If you don't have one Join A Supergroup! I havn't run into a Supergroup without at least 1 level 40.
Head to St. Martial and have your level 40 friend start running through the paper missions, DON'T TAKE KIDNAPPINGS, you'll want "find that item or kill so-and-so" type missions.
Have everybody team up with your 40 friend and have him enter the mission. NO ONE FOLLOW HIM. After his entry everyone will need to quit the team. He should use his stealth to run to the end of the mission to kill the boss or grab the item.
He will invite everybody back on the team and you should enter the mission. If you have extra people who need in have some people quit the team and invite those waiting outside and have them enter as well.
Your 40 friend should then complete the mission. You'll get 5000+ xp.
Everybody exit and Rinse and Repeat.
Note: I did this at level 24.
With the new update they tried to nurf this by making it so that you could only gain experiance if you were within a few levels of the person who started the mish to begin with.
To get around this have someone near your level start the mish and then do what I said above.