*D.L.C. ONLY*
I have a method that will net you 100 kills in about 5-6 min. You'll need a second controller as this is done in split screen co-op. It can be done with just you and the AI, but you will get between 70-90 kills in about 3-4 min. instead. Weapon customization and loadout is important to achieve the best time, so I will list the optimal settings as well.
Start a split screen game on casual difficulty and use the chapter select to load "The Saboteurs" chapter checkpoint (second to last checkpoint of Chapter 9), the mission will begin with a short cutscene detailing the objective to stop the train. Once this is done, move both you and your partner into the small alcove directly behind you and enter the weapon customization menu. You will either want to upgrade the M416 (primary) or the AK-47 (primary) but they both have equivalent stats when maxed out properly.
Equip the following:
Primary - AK-47/M416
Barrel - SC-4 TITYOS
Stock - HL Reflex Core
Cartridge - AK-47 30 Round Mag.*
Front mount - Slanted G5 Grip
Suppressor - E3 Muz. Enhancer
Scope - Red Dot [3X]
Secondary - Desert Eagle (DE-0.44)
Suppressor - SV5 Muz. Reducer
Special - You probably won't ever need to use, just in case pick your favorite snipe.
*although your clip size is smaller your damage is increased which is much more essential, and you still will have the same overall amount of ammunition.
Now that the customization is done, exit (don't worry about customizing your partner, his kills don't count toward your total) and quickly move your partner behind cover of the first column to the right, outside of the alcove you are standing in. Then you will want to move yourself outside of the alcove to the stack of boxes on the left. (MAKE SURE TO STAY ON THE DOCK!)
From here I recommend switching to your desert eagle and using it to snipe the enemies in the alley in front of you. There are other enemies across this section, but you will want to focus on the ones directly in front of you to maximize the time spent. Time your shots carefully and you should only need one well placed round to take down an enemy. Every so often an enemy will get on the turret so be sure to take them out as quickly as possible, also occasionally an enemy will run and hide behind a tall stack of boxes to your left. Be sure to keep an eye out for this and try to shoot them before arriving there as it can be difficult to shoot them while they are in that position. Feel free to alternate between your primary and secondary at your leisure, just be sure not to use ammunition frivolously.
You can check how many enemies (per session) have been killed by pressing start at any time and looking at the bottom right of the screen. 100 is the maximum that can be killed in this area, so once you have hit this number you will simply want to restart the checkpoint and repeat. When you do this however your changes to your weapons are reset, so you can remedy this by having your partner step jump you over the wall and getting a checkpoint first and then you will need to quit and select the chapter over again. After doing this once the changes are saved and you can restart when you get done killing 100 enemies.
Sorry for the long read, I like to be detail oriented. Hope this helps!