Settings: Start at Wave 20
Host Selects Map
Mutators Enabled (Super Reload, Instagib and Infinite Ammo) [Not necessary]
Make sure the host chooses Blood Drive and everybody is either Commando Dom (LE)
or Mechanic Baird. Now this where it's different from other solutions. This is assuming you have five players:
You're going to stop other bosses from spawning in by standing in their spawn zone. Each of the five players should head to the designated area and stay there until the boss spawns. Make the command post that is closest to spawn.
1. First player stays touching the wall outside of the frag grenades closest to the command post.
2. Second player stands on the left side of the sniper/torque, facing the weapon.
3. Third player stays in the hallway of where the grenades on your spawn are.
4. Fourth player stays in a few inches to the left of the opposite side frags and a couple feet from the entrance to the hallway.
5. Last person stands halfway between 1 and 2.
Map for reference:
Red is the command post that should be bought. Blue boxes are where the players should stand. From the top blue going clockwise this is: 2, 4, 5, 3, 1.
If done correctly a Lambent Berserker should spawn. In a boosting session my group and I tried this and it worked both times.
Make sure you don't die as buyback is $3000 and if you are not alive at the end of the wave you won't get the achievement.
If you want me to elaborate on a position post a comment telling me what position.