In Level 10: Reclaimed, when the objective is "Destroy the AA guns (3 of 3)", you will reach
a point where a solider is using a flamethrower to burn an alien, and "Checkpoint Reached" 
will appear on the left side of the screen. In this area, simply go down the hill and kill 
all enemies, then reload the checkpoint, and repeat this as many times as desired. You can 
get 6,000 XP an hour and complete many challenges for additional XP using this trick. You can
do this at any checkpoint in the game, but this is a really good area to kill a lot of enemies
and get XP.

In Level 10: Reclaimed, once the third AA gun is destroyed, you will continue through the rocky
exterior to a destroyed building. The floor is destroyed just past some ammo to the right of 
the entrance. Stop before leaving this building. There is a sheer drop after stepping through 
the exit door, leading into a large open area with unlimited spawning aliens. From the exit 
looking out into the courtyard, look to the far right. Equip any scoped weapon, and look for 
the aliens appearing above the spotlight. They will run across the roof before jumping down. 
If you stay inside the building, and do not drop down into the courtyard, the aliens will never
stop spawning. Kill as many aliens as desired to get as much XP as desired and complete 
challenges. 
Note: When you run out of ammo, simply reload the previous checkpoint. You will retain any 
points gained toward challenges and XP.