Standing On My OwnG 20

EDIT: 4/26/14. This soultion was written on launch and while some of the info is good, some of the info is out of date. Refer to the rank 50 guide for more help.


Legit Method:

If you are just starting this game or pretty new and having trouble leveling up then I suggest using the cleric as your main character.

The mage and the warrior are offensive characters so you might have trouble against all the veteran players out there with their high stat gear.

With the cleric you can just sit back away from the action and gain XP by healing your team, giving them sacred armor, and reviving them when neccesary.

Don't forget to use your team skills at every possible chance too as they can give up to +64xp for a full group.

Constantly spamming all those powers it was not uncommon for me to be the top scorer in a match while only having 1 or 2 kills.

Of course, once you leveled up a bit and grabbed yourself some nice gear you will probably have better results using a warrior or mage as your main character.





Boost Method:

It is entirely possible to boost this in co-op mode but I do not recommend it and it takes a full lobby of 15 people.

Basically you capture all the control points and make your way into the enemy castle.

Just leave the big tower alone and start killing the enemies.
They will keep respawning on the floor of the castle and are easy kills.

Launch team skills for even more xp, tornado in particular will grant a player a huge amount of kills/xp.

When the time is nearly about to expire make sure to take down the big tower for a 200+xp boost for everyone inside the castle.

You can get up to +3000xp in a 15 minute match.

Credit to the jerks who showed me this method by using it against my team in a MP match.





Conclusion:

In total you will need 24,400 xp to reach rank 20 *thanks TheDarKnyht for this info*

No matter which method you go for it will take you anywhere from 5-10+ hours in game to get this. However, because of how long the matchmaking takes, your hours actually spent in front of your xbox will be far greater.

If you do plan on completing the game then reaching rank 20 is not the final hurrah as you will need to hit rank 26 to unlock the final story mode level so you can get this following achievement:
It Worked