Gears 5

Using this method, I was able to get 4% (16 Million) every 10 minutes.

You'll need Blademaster at level 15-20 to do this solo.

Equip these cards
Brawler
Venom blade
Shock shield
Thrill of the hunt
On the flank
(If you are grinding this achievement with others, you can replace Thrill of the hunt with Brutal claw and progress through the hive until you find a claw.)

Start the escape hive called The Wanderer and put the first modifier on.

When you spawn, stand by the door and wait until the venom detonates, open it and lure the invincible scion from the right into a corner and melee him until you are able to spam your ultimate. (If you are doing this with others, find the saferoom and kill an enemy with a claw, there is another scion to the right of the saferoom exit.)

If the achievement stops progressing, go to the start menu and switch profile to any other account you have, wait for a few seconds and switch back, go to Customize and wait for it to load. If that doesn't work then you'll need to do more damage and try again later.

Quadrisecant has an incredible solution below. I wish I would’ve used it when I was going for this achievement.

Total annihilation: Deal 400,000,000 damage in horde or escape

WARNING: This achievement has been know for tracking issues. It’s been confirmed that if you have no progress on your percentage you will need to do significantly more than what the tracker shows. As long as you continue playing, you will eventually gain progress

Statistics
With this method I was averaging about 7.5-10 million damage every 15-18 minutes

Requirements
Striker class with most of the melee cards. Most preferably Melee Rush, Blunt Instrument, and Stim Siphon

Setup
Start up a horde game
MAP: Atrium
MODE: Boss Rush
DIFFICULTY: Inconceivable, survivor modifier off
BOTS: OFF

Pick up the fabricator and bring it slightly into spawn (This will give you some breathing room if you have to buy another breaker mace). Do NOT buy a breaker mace yet. Once the wave starts check to see what boss spawns. If it’s not two Carriers then kill yourself and reset the wave til you get them. Upon seeing the Carriers spawn, buy a breaker mace and stand near the gap in the wall in front of the spawn area by the barricades. You will basically be playing ring around the rosie with the two carriers, smacking one at a time. Depending on how high your bleed card is, you will have to stop hitting the Carrier around half health to prevent them from dying. You can punch just one carrier if two is hard to handle. The process will just be significantly slower. If you do accidentally kill a Carrier, I’d recommend taking its power and killing yourself til you get another Carrier spawn

Stim and ammo are very important here. As long as you consistently melee for stim, it shouldn’t be a problem. You can use a heavy attack to create a shockwave and get stim from afar if you are concerned about dying right away. Make sure you use the weaker/light melee attack overall though as it uses less ammo. I wouldn’t recommend using your ultimate. For me it would just kill them too fast. You must keep an eye on your ammo as well. Once you run out, the mace breaks. You should be able to cycle two ammo boxes but there are three if they don’t respawn fast enough. Use TAC-COM (LB on XBOX) to see where the boxes are. You have enough money to buy one more mace on solo or you can add another player to allow you to buy more. You can do this for as long as you possibly can handle it and it should track in game while doing it

RECAP
Start a Boss Rush on Incon and get two Carriers to spawn. Bait them and wack the crap out of them for as long you can handle it. Just make sure you keep an eye on ammo and boss health

Here’s a video of me showing ammo runs and how to get out of a sticky situation. I also show the scoreboard so you can get an idea of how much damage you will do.

The only method I’ve found so far is to play Horde solo on river on wave 46 on Elite difficulty with bots off and these mutators More health, More lethal, Aggressive enemies and Regeneration. Play as an Engineer with high cards for repairs and reduced fortifications cost and stim on repair.

Set up in the barn with three turrets level 2 and 3 and barriers level 1 and 2. Play till the wave is almost over then collect the power as it drops then die and restart wave.

The best run I had was 850.000 in a 3 minute run, this way I was able to get 6 to 8 million in 2h to 3h. If you get a wave with Lambent then just run out and die, as they do way too much damage to the fortifications. Don’t forget to repair during the wave and after the wave. Also upgrade your perks as well they really help out.

Here is two clips just to get an idea of the set up. But at the end of the day set up however works best for you.