Age of Empires II: Definitive Edition

The mangonel is built from the seige workshop. Just play a random game againt an easiest level opponent. You want to play as the Celts as they get a 25% fire rate bonus with the mangonel. Put the game to infinite resources on post imperial age. Make sure the game is on fast speed and turbo mode is ticked to make it go even faster.

First spam some villagers (10-15), build some houses and 3 or 4 castles. Once the castles are built set the rally point for all of them to the same point. Next build a siege workshop and build 2 or 3 siege mangonels. Next spam elite woad raiders from your castles. On turbo mode they appear almost instantly. Hold down Shift+Q to build 5 at a time. They will all gather at the rally point. Next have the siege mangonels attack the ground where the woad raiders are rallying, make sure the really point is far enough away from the castle to avoid any splash damage. Then just keep spamming woad raiders until it pops as the achievement counts killing your own troops. Takes about 5 minutes and the enemy wont attack you in that time.

This can be done solo using your own soldiers.

Build a military building and set a way point

Build a Mangonel (or any of the upgraded versions) and set the ground attack (in the commands box left-hand side) to where the way point is.

You can combined this with any of the "produce X number of X soldiers in a game" achievements.

Mangonels are the catapults from the Siege workshops. If you see Onagers that means you've upgraded them but they'll still count for this achievement

To boost this, utilize the Attack Ground option with a couple of them and have your boosting partner feed you enemies by placing a waypoint marker from a couple of Barracks.

If you enable Full Tech Tree then Eagle scouts will produce really quick