Flame Maniac
 Zealot Build - Warhammer 40k: Darktide 

I highly recommend the 5% crit chance perk on the flamer, the others are flexible. This build relies on crits with the flamer, which combining that perk with blazing piety will be happening all the time. 

I made this build to get the 40 kills on stunned enemies within 10 seconds achievement, but ended up really liking it. You could switch the stun grenade with the immolation grenade, that's your preference.

The flamer requires no reloads. You'll be critting enough that you will expend all the ammo without needing to reload thanks to everlasting flame, and blaze away turns it into a napalm launcher. Everything melts. The only downside is that you're out of ammo really quickly.

Hammer is self-explanatory. Chain heavies to clear hordes, overhead special for elites, overhead specials to destroy bosses. Fury of the faithful will almost always be available to you. To the point that you could replace redoubled zeal with another talent if you want. Once blazing piety is up you'll always be critting. Throwing enemies around with the thunderhammer allows for backstabs. Both provide cooldown reduction.

The only thing this build can't do is deal with enemies at long range. Gunners won't be much of a problem; hide for 2 seconds until FOTF is off cooldown, charge them and beat them to death. Snipers are the real problem. If they're somewhere you can't reach, you'll have to rely on a teammate to take care of them.

If you use the copy/paste talents mod: 

/usetalents zealot 15;5|1,6|1,7|1,8|1,9|1,15|1,19|1,20|1,23|1,29|1,30|1,31|1,38|1,41|1,43|1,44|1,45|1,53|1,57|1,58|1,59|1,65|1,66|1,71|1,73|1,75|1,77|1,78|1,79|1,80|1