THE Zealot Knife built (momentum)
 Zealot Build - Warhammer 40k: Darktide 

Stabbing? Stabbing! And burning!

Yes, the dueling sword is probably the most overpowered melee weapon right now. But combat knife is still more fun. Sprint around and murder every elite and special you can see, and then whip out the flamer to torch hordes and bosses. This build is extremely survivable, and rewards precise melee and dialed-in defensive/movement mechanics better than any I've played. 

This build works best on Auric Maelstrom or HISTG damnation since it's easier to keep all your buff stacks up with that enemy density. It's capable of one-shotting any normal enemy, unclusing crushers, provided you have stacks of all your buffs up, which is pretty much always the case during an auric maelstrom. 
 

Keys to success with this build:

• pop stealth constantly to one-shot basically anything with a backstab. 
• always be dodging to achieve backstab hits for toughness regen and cooldown reduction and bonus damage. 
• spam up to four push attacks to deliberately deplete stamina to proc the +20% melee dmg for 5 seconds. this can be done as you enter stealth and position yourself for a backstab crit to deal INSANE damage and one-shot basically anything. it's possible to hit monstrosities for like 20-30k dmg like this
• Flamer nearly never needs to reload so long as you only pull it out when there's several enemies to hit at a time. Everlasting flame and crit rate ensures that you can fire basically continuously forever until you run out of ammo entirely. this means you can whip the flamer out for quick horde clear pulse real quick, then get back to melee... and still have it available again later without downtime to reload it. 
• Flamer also makes for good boss dmg or clear huge hordes of elites because of everlasting flames and blaze away allowing you to get full stack of +% power from continuous spray, and then just keep spraying. 
• Momentum stacks apply to flamer attack speed, so you stack burning super fast, which means you can delete hordes of trash with a very quick 2-3 second spray

Closing thoughts- I'm considering switching the flesh-tearer for something else since this build just one-shots most everything and the bleed stacks kind of go to waste. Maybe Precognition.