BabyishDuck's solution (
Solution for MW3 - Slaughterhouse In Call of Duty) is useful but I want to add a few more points. The Outlast contract (Satellite icon) is certainly the most efficient contract to grind out kills.
1. Pack-a-Punch (PaP) your RGL-80 Grenade Launcher ASAP.Beyond the benefits of higher damage, the key benefit of PaP'ing your RGL-80 is that the grenades will immediately explode on impact. A non-PaP'd version needs some distance before a grenade explodes, which makes killing zombies more difficult (but by no means impossible.) You'll want PhD Flopper when using the RGL-80, PaP'd or not.
2. Combine Outlast contract with the Act 1 Story Mission "Extraction".Assuming that you're starting the mission (or quickly obtaining) perks such as Jugger-Nog, Speed Cola, PhD Flopper and Stamin Up, and your weapon(s) of choice, you can get more use out of your perks and weapons by completing the Outlast contract then heading for the Act 1 Story mission Exfil - called "
Extraction". Keep in mind you need to factor in time to get from the Outlast contract to the Exfil location and keep in mind where the Aether Storm is placed. In some games from where you are to where you need to be will be opposite ends of the map - you'll need
at least 5 minutes to travel there by vehicle.
3. Use turret circuits in "Extraction".Once you interact with Dr. Jensen (through a locked door) and you eventually make your way up to the helipad, there is a turret circuit on the north west corner that you can deposit turret circuits into - up to 4 at a time. Each circuit gets you 2 minutes of absolute killing power where you can basically sit there and do nothing if you wished.
There are at least 8 guaranteed drops of turrets, although you can sometimes find a few more which are placed randomly towards the north end of the map. I would recommend not spending too much time looking though - these 8 drops give you 16 minutes which is plenty - and you may waste more time looking for circuits which aren't there rather than killing zombies.
See map:
3a. You can place sentries at zombie spawn points too.I found that at least in Extraction (but this should work for contracts in general), if you can identify where zombies are spawning, you can drop a sentry gun nearby and it will kill zombies for you too. This is handy in Extraction as zombies spawn points are often outside of the direct point of view of the bigger turret (the one you deposit circuits into). You can also just camp at the spawn point while the bigger turret does it work and keep killing zombies as they crawl out of the ground.
By doing both Outlast and Extraction - 90 to 120 minutes in all if you maximize the time - you're almost guaranteed to get 3000-4000 kills, especially with using turret circuits. While this may not be as much as other solutions are suggesting you can get (I was never able to get more than about 2100 kills in an Outpost contract myself), it is a consistent and reliable strat.
4. Bonus: level up weapons easily!If you go to the Extraction mission and use the turret, while it's killing several zombies per second, the weapon in your hand is earning XP (assuming it isn't already at max level). I've max leveled several weapons doing this.
With the suggestions above and some practice and familiarity, you should be able to clear this in around 15-25 hours if you're starting from 0 kills. But to be realistic, if you're starting this game mode for the first time, you will need more time to grind though Act I's missions, obtain schematics, etc. Like many others, I had my progress wiped by the stats reset on December 7th 2023. It took me about 100 hours to get 45,000 kills the first time (although I was working on other achievements along the way), and 45 additional hours to get the required 50,000 kills after the stats reset.
Good luck, and good grinding!