This achievement is not difficult, I did yesterday with my team on the 2nd attempt.
You need to know the melee combo that is very important to execute drones: press B, B again and hold B and the character will do an animation where it will kill the enemy even if it has more than half life.
Charachter we used are: Kat, Emile and Lahni.
You need level 4/5 cards to get it.
Kat is very important for the Ultimate, she uses a hologram to distract enemies and then skip parts.
Emile have bleeding damage melee
Lahni has high melee damage
The map where we did it is Hive.
Strategy: When you start and open the door you don't run, just walk very slowly otherwise the enemies will notice you, doing this you can kill the rejects and drones without problems.
Take the road to the left and continue to kill enemies with executions and melee combos in case they notice your presence.
As soon as you have killed everyone in this part there will be 3 supply rooms, one player will have to have the incendiary grenade, another will have the frag grenade and the other 2 flashbangs, if in case he only spawns a flashbang the player will have to take the ammo explosive box for have the second flashbang.
As soon as everyone is ready a player will have to go ahead and shoot on the drones and scion for aggro them, as soon as he's done he'll have to go back to the area where the 3 supply rooms are and wait for the scion to come, probably 2 or 3 drones will also come. As soon as the scion is near you the player with the flashbang will have to stun him (only 1) and after all players melee him , kill the other 2-3 drones that will be close to you, done that too you will have to head later and there will be the round room where there will be so many drones, here you are being repaired and head to the right and wait for the player with Kat to have his ultimate. As soon as he has his ultimate ready go to the left where you entered and the player with Kat must activate hologram and everyone will have to run on the left side, almost at the end where there is the safe room the player with Emile can use his shield final to protect the team from the shots of enemies not distracted by Kat's ultimate, once that is done you will be at saferoom.
The player with the incendiary grenade can take if he wants the shock grenade but it all depends on what types of enemies there will be as soon as you exit the door.
These are the 3 cases of enemy spawn:
- Juvies and Sire
- Mulcher Scion and one Grenadier
- For the first case which will be the worst of all the player with the flashbang must stun the drones and the player with Lahni will have to use his ultimate and kill all the enemies.
- For the second case the player with the shock grenade will have to use it to slow down and hurt the juvies and the player with frag granade will have to use and kill as many juvies as possible, the remaining enemies kill them with the melee and be very careful to the Sire.
- For the third case the player with the flashbang will have to stun him and all the players melee him, once killed think also to kill the grenadier.
Once you have passed this part too you will have to go to the right and there will be a couple of drones with sniper and hammberburst, stay to the right and kill all these enemies as quickly as possible using the melee combo, once killed all go straight ahead and in front of you the Warden or Swarm will spawn.
If you spawn the Warden the player with the flashbang must stun him and the player with Lahni must activate his ultimate and hit him from behind and kill him and take the hammer. But if in the first part you have already used the Lahni ultimate for the Grenadiers you will have to go ahead and there will be 2 supply rooms, your task is to have one player go each supply rooms and the warden comes to stun him with flashbang you will find in the rooms and leave immediately and close the door so he will be locked inside.
If instead you were to spawn the Swarm you will have to do the same thing and close it inside but without stun him because it will have no effect.
Once that is done, kill all the enemies that will be behind you and especially the Sire and take the flashbang and shock grenades inside the supply on the left.
Now there will be the last part before the extraction zone go to left behind covers and if there will be drones close to you kill them with the melee combo or player with Lahni use hammer, NOT SHOOT in front of you to get the scion's attention, as soon as they are on the left side the scions immediately take the road to the right and if you find some enemy in front of you kill them with the hammer or stun them specially the scions, while you run towards extraction zone the player with Kat must use his ultimate so most of the Enemies including juvies that will arrive while you run away will be distracted by the hologram,remains enemies that will coming kill them and just finished and you will be save achievement will pop up.
Guys I hope I have been useful to you, soon my friend's video guide on youtube will be uploaded and i will post it when available.