To earn this achievement you’ll need to play in the Multiplayer mode of Bannerlord. There are different game modes within multiplayer such as Skirmish and Captain, and you can play these in a competitive environment, or you can start a custom Captain match with AI companions. Defeating 10,000 enemy troops may seem like a lot, but if you consider that any kill from your allies counts towards this, it doesn’t take very long at all if you’re willing to play Multiplayer consistently. To speed things up you can use things like trebuchets to get multiple kills at once, so you should always consider ways to gain numerous kills with the least amount of effort if all you want to do is earn this achievement and then quit playing the Captain mode. In truth, you could just as easily join a Captain game and sit there doing nothing while your allies earn the kills for you, but where’s the fun in that?!