Push attack spam
 Zealot Build - Warhammer 40k: Darktide 

You can chain push attacks on the mk6 knife super fast, and with 2 +3 stam curios, Zealous , and No Respite, you can chain them into a target with no delay at low stamina. 

This is because you have 10 stamina bars, so no respite always gives 1 full bar back. Normally a mk6 knife push costs 1 stam bar, so with zealous it only costs 0.85. Since you can always push even at 1% stamina, the first push attack drops your stam to 2% and lets assume it wasnt a crit (you will have 80% crit chance) and so no respite didnt proc or it was on cooldown (1s cd between procs). you can then push attack again immediately and get a crit and repeat. This will keep desperation at 18-20% extra damage, and Retaliatory Defence is there so you never get block broken. 

Vicious Offering actually works on the combat blades push attacks and the light attack that comes out after weapon special punch. Your hordeclear will usually be H - H block cancel so it helps there too. 

 Precognition is an excellent choice. You could also run Riposte for 100% crit chance. Dumpstat for the knife should be penetration if you are running Uncanny Strike , which imo is mandatory. The difference in dps between a 60 pen knife and an 80 pen knife isnt discernible for me in the psykanium providing that uncanny is stacked, which it practically always is. The added dodge distance and sprint speed from maxing out mobility is really nice. 

I like this shotgun to supplement my hordeclear and elite pack killing, just shoot special flame rounds into packs and Manstopper will give it infinite cleave (still stops on carapace). Throwing knives inherit whatever weapon blessings the currently held weapon has, so with manstopper and scattershot you can get infinite cleave throwing knives which is pretty funny. you can also use the regular rounds for specials/gunners. You could whatever ranged weapon you want, just if you take boltgun or flamer I recommend taking throwing knives to cancel the equip animation. Also Against the Odds works on all damage (dots, ranged, and melee) and the cleave works on your ranged weapon which can help to get scattershot procced. You can light bulwarks on fire by shooting their shields and the fire shots have a ton of suppression so you will suppress even reapers with one shot. Burn also has very high unyielding damage so you are pretty good at taking out reapers and bulwarks and keeping a boss on fire if your team lacks boss dps.

Flame grenades are really nice with uncanny strike but feel free to use whatever nade you want. I think all 3 are viable and boil down to build/preference.