You get achievement for the win in the game with 8 people

Go multiplayer
Call some people for help
Host a game
Pick an 8 player map
Split into team 1 and team 2 (digit in the square)

Then
1 - Ask a player to assign the team 2 , or assign him yourself and ask him to leave the match after the start. All team 1 will get the achievement afterwards.
If that team 2 player doesn't have the achi, then help him to get it in the next match (host it again).

To provide achievement to all except you:
2 - As a host you can't leave. But you can join team 1 and send the rest to team 2.
Tell them to kill your lord. Build a Cathedral near your castle to be found easier.
This way all in team 2 will get the achievement by winning the game against you.

Till the Crusader DE has not released this game has pretty active online.
The lobby built the way you don't see the started games, therefore it looks always empty, but it is not, and there are people meeting it.

So name your hosted server as "1vs7 need help". And mind to do it in the prime time
People will join, lots of them also dont have it. Hence it is enough to organize about 3 people, and the rest you will find quicly this way.

p.s. There are also will be people joining, that want a real game, or call you to their server. Clarify your purpose, ask to leave and kick, repeatedly.