Age of Mythology: Retold

15 minute method with the ingame editor

1. Start the game, click on the = (top right) and select editor.
2. After a loading screen you see a green and empty map.
3. Click on objects (top row in the menu bar) -> place objects
4. A new window opens. On the left side select player 2 and on the right side you can select the object type. For boosting the kills a Militia unit is enough. Select it and place a lot of units on the map (close together). I suggest to place the units close to the north side of the map.
5. Optional: you can use the copy & paste feature to make things easier after you have placed a good chunk of units. For this click on button #12 (top row below the menu bar) to go into copy mode. Now select the area with the units to copy and press CTRL + C. Now press CTRL + V and a new window opens up. Make sure the checkbox for units is active (to copy units). Now with every left click you create a copy of the units.
6. Warning: Don't create too many units. I did it with about 2500 and the performance was pretty bad. (That is also why I told you to create the units at the top of the map because when the game starts the screen is at the lower part and there is no need to move the screen. So you save a bit of performance.)
7. After placing enough enemy units place some strong units for the player, like a titan, for easy kills. Select object in the top row -> place objects. This time select player 1 and search for titan as object. Now place about 5-10 units for player 1 (close to the enemy units. This way the titans attack automatically.).
8. The last step in the editor is saving. Click on file -> save (or click on button #3; the little disc icon) and type in a file name.
9. Now back out into the main menu of the game (file -> back to main).
10. Select skirmish and as game type (top left) -> custom scenario. Select the file you saved and start the game.
11. The titans will automatically attack the Militia and when all enemy units are dead (or your units defeated) you can view the post game stats. This way you can estimate how many times you have to load & complete the scenario for the 10k kills.

I got this through natural play, pursuing the other achievements. I got it around the time I was finished with 30 scenarios in the main campaign, all scenarios of the Golden Gift campaign, three of the Atlanteans campaign and having done some cheevo-focussed skirmishes.

If you need this more quickly, for a challenge f.e., do the following:

Start a skirmish with as many AIs as your PC can handle on the lowest difficulty. Choose any god you'd like. I choose to do it with Anubites (so either Isis or Set with Anubis as the first god chosen), but any sufficiently fast unit with decent attack will do. Set ressources to infinite and if you are comfortable with it, increase the speed of the game.
At the start built a temple and as many houses as you can. After the first temple is finished, age up. Should you decide to use a mythical unit for this, built more temples.
In the second age, built a forge and if you want to do it with common human units, built their respective recruitment building. Age up and research everything that helps your chosen unit.
In the third age, built a fortress or many, if you want to use those units, age up. Again, research whatever strengthens your chosen unit.
In the last age, auto-queue whatever unit you have chosen. Once you got your 100, circle around the map, attacking the respective AIs. Pay attention that your units do not destroy the town center. Once you killed all the units, move on to the next AI. Once you have circled around the map, you can start over at the first AI again until you achievement unlocks.

The AI will not attack your base, so you don't need to worry.