Chivalry 2

Sorry but this no longer seems to work. Switching teams is not longer possible to do ruing the starting charge. Hopefully this will be changed at some point. It’s sad because I prefer defending than attacking regardless of achievement hunting. This game seemingly needlessly restricts the player from practicing HALF OF THE GAME. It makes no sense. But playing attacking with bits on the lowest amount can actually make it very easy to complete but it’ll take more time.

So this is simple but very time consuming. Select “Play” then go over to the “Offline” tab, and select “Offline Practice.” Here you can choose any map you need wins for. Now as far as I know, you’ll always start on the offensive team, so pause the game and hit switch teams and select the defensive team. And lastly just defend. Most of the time it should be very easy to defend the first or second objective. At the end of the match you can quit to the menu and select the same map again, and repeat until your achievement pops! I wish there was a quicker way but this is by far the easiest to guarantee a win instead of gambling on wether or not you’ll have a competent team.

Step-by-Step summary

Select “Play”
Select “Offline practice” in the “Offline” tab
Choose the map you need wins in
During the intro, pause and switch teams
Then defend the point
After victory, quit to menu and repeat these steps

This is my first guide so I hope it helps!

Works for all maps.
The quickest method I found is to go to:

Play -> Stand Alone Server -> Find the map you want (eg. Dark Forest) with 0/42 players. You cannot force a map to appear as far as I'm aware but they do come quiet often, if the map isn't there just wait half a minute and refresh the search with . Switching between maps that give achievements is ideal to prevent waiting for specific maps (Coxwell, Dark Forest, Rudhelm, Lionspire).

This will put you in a game with 6 bots on each team. Press start -> change team and make sure you're on the defending team. For Dark Forest it is Agatha, the blue team.

Defend the first objective (takes 1-10 minutes depending on the map and how far it is in the game when you join. The bots wont cause you any trouble as they're quiet easy.

You can also do this offline like others have suggested but I found it to take way longer because you cannot change teams and will have to win as attackers.
Good luck!