This is my very first solution on this site, so make sure to give me some constructive feedback so I can make it better.I've gotten quite a few messages on Xbox Live regarding help with this achievement so I figured I'd write a guide instead of writing to everyone separately.
Part 1 - Class SetupI did this with one other player (credit to stulen). We were both using the Medic class and we both had the 'Medic Upgrade' that you can buy from the armory with teeth. This upgrade is
very helpful, but it is not at all required (it only costs 15 teeth, though, which it is absolutely worth).
Note: I just escaped Hardcore with 3 players and it honestly wasn't any harder than doing it with just two. Therefore no matter how many people you are, this guide should work fine.Part 2 - Hardcore DifficultyOn Hardcore difficulty you only gain skill points for completing challenges, which means that you should do your best to complete as many of them as possible. This also means that you'll only have enough points to upgrade the most important stuff. (I'll get to that in Part 4).
Part 3 - Exodus (Skippable Areas)In Exodus, you actually don't have to do all of the areas. The only requirement to reach the final area is to defend all of the generators, which means that the areas blocked by gates are skippable (between the fuel station and the garage, and the "hobo area" between the spawn point and the three story building). If you skip these you will miss out on two challenges, though, leading to less potential skill points. I can confirm that skipping these areas doesn't void the completionist achievement (credit to NIGHT FURY444).
Part 4 - Relics and Skill PointsWe both used the 'Limited Ammo' relic, which we found the best for us. This mostly comes down to preference, but I would recommend 'Limited Ammo' as it worked perfectly for us.
We upgraded our classes first, but only to level 4 to avoid scorpions' gas damage. Then we upgraded Armor and then we put our remaining points on Sentry Gun.
When using 'Limited Ammo' upgrading your Ammo Type is completely useless, as you get the same amount of ammo with level 1 ammo as you do with level 5 ammo.
Our Loadout:CLASS: Medic
PISTOL TYPE: MP-433 Grach
AMMO TYPE: Incendiary (Higher quality upgrade)
TEAM SUPPORT: Armor
STRIKE PACKAGE: Sentry Gun
EQUALIZER: Crowd Control
The achievement pops for every player once you have escaped. At least it did for us, not sure if you have to actually escape. It's also worth mentioning that we only killed two of them, if you detonate the thing at the end they all automatically die.Thanks for reading and good luck!