Shot Slasher
 Zealot Build - Warhammer 40k: Darktide 

The main focus for this build is on fury dash while giving you and allies some decent toughness tanking. The method of such is a braced double barrel shotgun blast followed by a chainsword rev, fury dash and wail on the enemy with a heavy attack, dodge away and repeat from a rev, if you have the time, reload and start from a double blast shot again. This method tested myself will ruin almost everything aside from bosses and bulwarks(on a shield hit with the shotgun). It's a high pace fast reflex set up, it will take some practice to master the method.

I would recommend the Tigrus Mk XV Heavy Eviscerator due to both of its heavy attacks being single target while having all light sweeps but it also works with the chainaxe and assault chainsword. You can test it with other weapons but all of the saw weapons have given myself the best damage results since you can still dodge with a revved hit without losing more than 2-3 ticks of hits from it due to the saw ticks having a longer hit range than the weapon itself does after first contact. 

This also has a much heavier single target damage with thunder hammers but you will sacrifice horde clearing potential but in return only bosses can withstand a single hit.