Map- Blood Drive
First some general tips.
You CANNOT revive people using their cog tags as this will void the achievement. I recommend that you never pick them up.
You must do this achievement on the 25 wave horde variant. I recommend horde lite. I have received reports from my friends that they did not receive the achievement on a 50 wave horde mode. Plus, 25 waves is less than 50 waves, so don't make your life harder.
You CANNOT purchase anything from the fabricator. That means you cannot buy weapons, fortifications, class items such as the hammer of dawn, team revive, etc.
You CAN deposit money into the fabricator (there isn't a point to depositing money unless you want some extra points).
It is okay to fail a wave. In fact, you can purposefully fail a wave to resupply your grenades/weapon ammo and to change the boss (for further information regarding the boss please see section 7).
You must play on INSANE. This will not stack with inconceivable.
If a player gets disconnected, IMMEDIATELY pause the game, reinvite the player, and continue the match when that player is fully loaded into the match. We had a player get disconnected on during the countdown to wave 25. He rejoined and still unlocked the achievement. He was only missing for 10 seconds during the countdown for the wave.
1) Decide which classes you will run. Any combination of soldier and heavy will work. Having 1 sniper would also be helpful.
I recommend either
4 soldiers, 1 heavy
3 soldiers, 1 heavy, 1 sniper
Recommended cards for each class are the following:
Soldiers should run grenade damage, grenade plants, grenade capacity, and resupply.The fifth card is up to you, but I would recommend either assault rifle capacity or assault rifle damage.
The heavy should run explosive launcher capacity, explosive launcher damage, and marked damage (especially useful against bosses). The next two cards are up to you, but we personally used heavy weapon damage and thick skin.
Sniper should run called shot and mark boost. The rest are up to you but I would recommend explosive headshot, headshot damage, and precision rifle damage.
As always, the higher your cards are the better. I would recommend level 6 green cards, level 5 blue cards, and level 4 purple cards at a minimum.
2) Decide which map you will run. The two best choices that I have had success with are blood drive and war machine. This guide will describe the process for blood drive (dlc map)
Make sure to be on INSANE! it will not stack if you do it on inconceivable.
3) Game mode. I recommend horde light. This mode has 25 waves with a boss wave every 5 waves.
The way to change your game mode is to press start in the horde lobby for match options, click on enemy list, and change to horde lite.
4) Fabricator placement. The first thing you should do on blood drive is move the fabricator to the diner spawn (if you are going down the stairs the rooms will be on your right) and place it in the doorway of the room. Just make sure you leave enough room to walk your character through the doorway. This will ensure that enemies cannot enter the room except for the carrier.
5) Grenade plants. Grenade plants help a lot. Make sure your soldiers place many grenade plants between waves. Also, make sure that you put enough space between each grenade so that one enemy doesn't create a chain reaction and blow up all of your grenade. Also, make sure to save a few grenades for later in the round.
I recommend putting some grenades just outside the doorway, throughout the staircase, the small room across from the doorway, the corridor heading towards the train station, near the ammo box between the diner and hospital spawns, and near the ammo box between the train station and triage room.
6) Player positions. Have two players hugging the walls on the doorway to the room. They should both be soldiers. Throughout the game they will be throwing grenades at enemies that are approaching the doorway. These two soldiers should have the highest level resupply of your group. When they don't have grenades they should use their lancers.
The heavy should be a little bit further back in the room, face the doorway, and shoot boomshots, drops shot, and heavy weapons.
A third soldier should be covering the windows. Juvies will climb over the windows, and it is their job to kill the juvies and warn the rest of the team that they are coming. A shotgun would be the best way to kill the juvies. Also be careful as the juvies tend to glitch through the right wall, so don't have the soldier hugging the right side of the doorway to the windows.
The fifth player (soldier or sniper) will ensure that enemies do not spawn in the room. If you are facing the doorway and windows the enemy spawn is on your far back right. The fifth player will also reinforce soldiers 1 and 2 at the doorway, and will reinforce the third soldier at the windows when juvies come if there weren't any enemies spawning in the room.
7) Choosing your boss. During a wave you can kill yourself in order to change the boss. The first 3 attempts of a boss wave will give you the same boss. After that the boss will change every wave. If you get a carrier kill yourselves until you get a different boss. Carriers are the only enemy that can get through the doorway, and will kill you all quickly. We found that the snatcher was the best boss to go against since it gets stuck in the doorway which means that the soldiers can throw grenades at it and kill it quickly. Swarmak would be the second best boss since it typically gets stuck in the middle of the map between the diner and hospital spawns. However, they take a lot longer to kill as compared to the snatcher.
Good luck with the achievement. I welcome your feedback and will make changes to the guide accordingly.