Isonzo

I recommend attempting this achievement on a fresh server, so from the main menu press RB so you're in the 'Play' menu. From here press Y to create a Custom Match, turn on the Friends Only setting, and choose the 6th Battle of the Isonzo Offensive starting on the Gorizia map.



Spawn into the game on Objective A as an Engineer on the defending team, the Austro-Hungarian Empire (the default is attackers, so press the emblem on the left side of the spawn screen to switch teams). Move to the front of the building and to the left you'll see the entrance to a tunnel underneath the train tracks. In the tunnel's entrance are sandbags and an HMG (Heavy Machine Gun) build point. Hold X on the green highlighted gun to build it and then hold X again to use the weapon. Now you just have to wait for the enemy team to walk across the road to either objective. I suggest only single pressing the trigger to conserve ammo (the bots go prone when the HMG shoots at them anyway, so it's easy pickings). Additionally, the bots shouldn't even bother attempting to stop you, and if they do, you can't be flanked here, so you'll mow them down anyway.

After killing 20 of them without dying your achievement should pop!

Go on dolomite, solo and all that. As an engineer build the machine gun a bit way behind the first objective at the base of the mountain. On the far side by some rocks stage right of the objective they have a gun bots will mindlessly man. The easiest way to identify it is to see a guy run up and mount it but you can see in this clip where it is. None of the bots will shoot at you so sit back, relax and knock them out.

https://gamerdvr.com/gamer/cream-khorne/video/165736064

Start a custom friend-only match of 6th Battle of the Isonzo on Carso. There is an LMG in the top left corner of the map, use that one. Just hop on the LMG and gun down anyone that comes up the beach. If you use Rifleman you can also refill the ammo. If TA finally uploads my screenshot, I'll add it to the guide for a better explanation.