Win alone against 3 human opponents.This is hard to achieve naturally and requires a decent level of skill or luck at the game. The best way would be to find three other people to help you with it.
Once you have your group, set up a multiplayer game with yourself on team 1 and the three others on team 2, Small or tiny map, all visible, can be any game mode but regicide is quick with a castle age start. Once it starts have the three people just walk their kings into your base and kill themselves, winning the game for you.
Once done swap places until you all have the achievement.