You must have the Havoc DLC pack. Load up Exo Zombies > Outbreak.
This IS possible solo, however is much easier in a co-op game with friends or randoms.
Simply survive long enough and don't spend too much money until you have 15 000 points NOT in total but in your current inventory - points already spent do not count towards the 15k.
In my experience the best way to go about this is get two good weapons, juggernog and that's it then simply wait until you have 15k. I've found the AMR9 in the circular room is a good point whoring gun and you can buy ammo off the wall :)
After you save up 15k it's worth then getting the "PC Load Letter" achievement if you do not have this already.
Solution for PC Load Letter In Call of Duty: Advanced WarfareTips to survive are:
-Start the low rounds shooting in the legs and knifing for more points
-Buy a cheap gun to start eg the Mk14
-Get to Exo Suit room ASAP
-Attain HealthBoost/Juggernog ASAP (2500pts)
-Attain two good weapons from the 3dPrinter/MysteryBox or the wall ASAP
-Train zombies behind you (gather them up behind you and run in circles around the room or map) once you reach approximately round 7 and upwards
-Upgrade your weapons using the upgrade machine whenever you can (2500pts)
-Pick up all powerups especially - DNA Bomb kills all zombies spawned in on the map, double points & instakill
-Knife/punch when you have instakill for more points
-Pick up all care packages! Why not they're free and they help you out!
-Gain free points from green security consoles by walking up to them and pressing x (randomly change around the map).
-Purchase all other perks in order of personal preference (I would suggest speed cola > quick revive > ground slam > gun-ho
-Prioritise killing zombies: acid > gas/infected > EMP > standard
Good areas for training are spawn and the circular room right next to spawn show on the map below.
Some additional useful info is contained in my map below!
Pop goes the cheevo!
Hope this helps
Happy Gaming!
-coreymawf