First off, the easiest mode to get this on is Infestation. Mainly because if you have the right strategy, and are confident in handling aliens en masse, your score will go up very quickly.
For a particular strategy, I recommend is Gateway (Thanks Guirec730) the one with the crashed space ship and the marine compound.
Anyway, go to the crashed space ship across the ledge is the sniper rifle, camp there grabbing regen ammo till you have something ridiculous like 50 sniper bullets. Of course this will not be quiet and you will most likely draw in some aliens, so try to stay alive.
Now when the numbers start dwindling head over to the mouth of the cave, and camp at the top of the mound. Most players arent too smart and will just run up the mound, so gun them down with the sniper rifle.
Check the cave periodically to make sure you wont get ambushed, although not many people take that entrance so concentrate more on the compound. It will take a lot of tries and luck, but you will get there in the end.
Good Luck :)