Hi, first of all i will tell you what you need to get this achivement rolling.
1 - You should get the DAY ONE GOLD M4 (2100 stars); if you want to kill 53k enemies getting this weapon shouldn't be a problem.
2 - Equip the weapon with low ROF (I am using Specter sight, high pressure gas sys., match trigger. You will not need a supressor for this, because it lowers the accuracy and the damage.)
3 - Equip your soldiers with max armor (crye precision airframe - helmet, spear RBA vest - kevlar)
4 - Your soldiers stats should have: 100 - accuracy, 100 - reaction and 100 - health.
5 - I will not specify the classes because any class will do just fine, but if you really want something "4 Direct Action" soldiers would be the very best, because of it's perks.

Enough of this, now it's time for the guide:
I will do this guide for Frozen Synapse players, if you don't have the expansion you can do this on Escape Plan -> Mexico -> Operation Mangled Antler (only 17enemies). The only problem is it will be a bit slower, because you dont have that many enemies to kill.

Now follow this steps:
1 - Select Missions -> Escape Plan -> Markov Geist -> Operation Hard Touch -> select Insane dificulty and press Launch:


2 - Now you will get your soldiers on the field. No need to select anything. See below.


3 - Press on the keyboard the numbers 1, 2, 3, 4, to select and ready your soldiers. Press them really fast for maximum eficency. No need to do anything else. Now click on Breach.


4 - Skip to step 5. Wait until the killing is done. Sometimes your soldiers die sometimes they live, it doesn't really matter. Wait until you see "NOW PLANNING PHASE 2".


5 - As soon as you see "NOW PLANNING PHASE 2" I found out that you don't need to wait to the end of the round for the kills to count. Somehow the game already knows how many enemies you are going to kill at the end of that phase. So... as soon as you see them shooting each other, -> press "esc" on your keyboard to see the menu. Press "RETURN TO MAIN MENU".


6 - You will return to this screen (below) and press Launch. Go to step 2 of this guide and do all over again.

This is the most efficient and less time consuming way to kill all 53,594 enemies. Why?
1 - First of all, you dont need to position your soldiers on the field. You start the game with your soldiers on the field.
2 - You have a open space with a lot of enemies to kill,
3 - You don't need to select and give orders to your soldiers individualy. Just press 1,2,3,4 like crazy.
4 - You will kill +/- 20 enemies each time...... >_> yeah! By the time you get the other achivements you will only need to do this more than 2000 times. But, the kills/time ratio is good.
5 - It's all about fast fingers and good computers. It took me 17hours to get it.

The only problem is that you never complete the mission because you interrupt it when you are going for the second phase. If you don't complete the mission you don't get the Seasoned Achivement (clear 1000 locations). A shorter match is advised to get Seasoned achiv.