I found this moderately quick method for this achievement. Load up Operation: Hydra; Mission A-1, then put the skulls Hollow & Pacifist on which will increase your XP income by x5. So your loadout should look something like this:
Primary Weapon: Assault Rifle
Secondary Weapon: Magnum
Armor Ability: Regen Field
Booster: Not Necessary.
Skulls Active: Hollow & Pacifist
This mission takes about 5-6 mins to complete, there is a small weapons cache at the start which consists of two Assault Rifles and a crate of Frag Grenades in it. With the Regen Field ability you can counteract the effects of the Pacifist Skull somewhat, however be careful how you use the Magnum because it may fire at a slower rate than the Assault Rifle, but it does a lot more damage to you, so if your health gets too low and you fire the Magnum you're dead.
At the end of the mission your XP income will be as follows:
Red Star Score: 10,000 – Income: 250 XP
Silver Star Score: 20,000 – Income: 500 XP
Gold Star Score: 35,000 – Income: 1000 XP
As you can see the goal here is to aim for a Gold Star each time to maximize the process, just repeat the process as much as necessary.
Hope this method helps & Good luck.