This achievement can only be earned in multiplayer, where the Skirmish game mode is available. There will be plenty of opportunities to kill your enemies in new and interesting ways, but you’ll need to focus on killing them with a melee weapon both unmounted and mounted, a ranged weapon both unmounted and mounted, and a couched lance all in one round. This should be fairly easy so long as you start the skirmish from the beginning and work towards it throughout.