Warhammer 40,000: Darktide

This is a hard one to give a guide for because they are constantly buffing and nerfing weapons but I generally recommend you bring any mass AoE ranged weapon like the voidstrike staff, Purgatus Staff and flamethrower and use them against the hordes of lesser enemies that appear notified by a large crowd noise cue. The higher the difficulty, the larger the hordes tend to be and some of the modifiers effect horde size as well.

I managed to get it on a "hi-intensity modified" difficulty 3(malice) mission at the end while playing as Psyker. A horde spawned and the enemies kept spawning from a door so a spammed voidstrike blasts into it as it opened until the achievement popped. I have the achievement on Steam as well and that time I used the Zealot's flamethrower against the back to back end hordes on the Power Matrix Repair mission (the one that attacks while the final door opens then the other that ambushes you as the team goes towards the dropship.

Having friends for teammates helps as they can deliberately not shoot the hordes maybe focusing on elites and specialists instead so they wont be syphoning potential kills from you.

Your best bet for this will require a Malice or higher-level difficulty and one of the new maps called the Carnival, where the level starts inside of a broken silo and you have to place a breaching charge on a brick wall, behind which will be 3 melee special enemies. You do not NEED it to be a high intensity mission, but it certainly does help to guarantee enough enemies will spawn in at once. The reason you want this particular map is because the place to do this Achievement is at the very BEGINNING of the map and not the end, making it easy to get to, set up, and rotate team members so everyone can have a shot at the Achievement.

As for loadouts, a high level or max level Zealot will be your best bet. Bring the upgraded flame grenades and an upgraded Flamer. You'll want to wait until the enemies have finished spawning in and they're built up in the small room where you placed the breaching charge. Toss both flame grenades at the broken piece of wall so the fire is spreading into the next room as well. Then ready your Flamer and burn anything still standing.

Obviously, to make sure you don't die before the enemies have finished spawning in, you'll want two of your teammates to play as tank Ogryn who are hopefully max level and have powerful shields and tank perks equipped. Their job is to just hold the shield up (but don't push as this can damage and kill the weaker enemies) until the enemy mobs have finished spawning and the room is full of enemies. Stand at the corner of the platform so you can see into the next room. To help with this, you might want your 4th squad member to also be a Zealot, but with stun grenades.