I find that the best way to get this is first by completing the entire story mode on Realistic. Once you complete the story mode you should be around lvl 21-23. To grind out the last 7 levels I find that the best place to do this is level 3.3. You should play this on Hard since you will not have to reset the entire level should you die. You should get anywhere from 2400-2900 xp upon completing this on hard. If you do it on normal I find that you get anywhere from 1800-2300 xp.
When you start up the level make sure you have the heal perk and the unblockable attack perk as these will help you speed through the level faster. Each run take about 5-7 mins. Try to focus on the objectives since ads will continue to respawn until the objectives(mini bosses) are killed. If you have a friend, you guys can speed run the round together as you can divvy up the work.