This achievement can only be obtained during the first Lair Defense "Protect the Lair" (main story) and is best done with 2 or more people, but it can also be done solo. Pre-requisite - Upgrading the Generator
(Not needed if doing Co-Op, but i recommend doing anyway to be safe)
After rescuing the engineer he will go back to the Secret Lair and stand by the generator (northwest corner) where you will be able to purchase upgrades and place traps, I highly recommend ONLY upgrading the generator for now as the other upgrades use power which could otherwise be used for traps. To upgrade the generator first you will need a generator plan which can be obtained though the side quest "The Assistant". At the end of the side quest you get to choose your reward from a generator plan, a trap plan, or some ability points, no matter which you choose you can always get the other two from the assistant who resides in the south east corner of the lair after the quest.
Once you have the generator plan, fully upgrade the generator and you can then start placing traps.Trap Placement
The trick with the traps is to use as many Monster in Box traps as possible, they grab the attention of the enemies giving you plenty of time to run to any part of the map and help, but this isn't needed very often as they are also very good at killing the enemies too! I also place a few spike traps at the parts where the enemies group up.
Machine Parts Required: 126
If you only upgraded the generator you should be able to get something like this:
Machine Parts Required: 96
If you upgraded everything at the engineer you should get something like this:
On Normal (solo) both of the above are fine, higher difficulties might want to play it safe and have more traps covering 3 and 4 so you can exclusively defend 1 and 2 at the Player 1 Co-Op spot.Co-Op Method
Pretty straight forward, stand at the co op spots shown in the above pictures and don't let anything past, if anything even reaches you that is. You can even go up to the enemy spawns and fight them there but be keep your map up and be ready to run back to defend the other spawn when necessary. After 3 Waves the boss will spawn at 2, simply pull the boss and his minions over to the group of traps at 3/4 (or stay at 2, up to you) and fight them there.Solo Method
For the most part nothing will get past the first lot of monster in boxes at 3 & 4 so you'll be spending most of your time defending the Player 1 Co-Op spot, but i recommend running back to the Solo area every time it's clear so you can be ready to catch that one sneaky guy that gets past your defenses. After 3 Waves the boss will spawn at 2, simply pull the boss and his minions over to the group of traps at 3/4 (or stay at 2, up to you) and fight them there.Other Details
There are 3 Waves, each consisting of varied enemies though nothing too difficult. Wave 1 Spawns at (1), Wave 2 Spawns at (2) then (3) and Wave 3 spawns at (1) & (2) then (3) & (4), the boss always spawns at 2.
Once you start the defense there is no way to restart it. Make sure to prepare before going in!
This achievement is Missable, there are two lair defenses in total but the achievement can only be obtained from the first one, if you fail you will need to start a new game or join a friends game to try again.
This is my first solution, be gentle :D