3.5-5% Strength for 3.5s on Hit. Stacks 5 times.
7.5-15% Weak Spot Damage. Weakspot Kills also ignore Enemy Hit Mass.
5-8% Strength for every 10% of magazine spent during continuous fire. Stacks 5 times.
Up to 2-5% Strength, scaling with remaining Ammunition. Stacks 5 times.
This is a flexible build with the primary goal of using your flamethrower to deal with hordes and your axe or another anti-armor weapon to deal with Maulers, Crushers, and other tough Elites.
As you will have anti-horde and anti-armor covered, that leaves you with two critical weaknesses: having range and being able to deal with Specialists. We solve this with the Blades of Faith blitz. Weakspot hits guarantee an instant kill on Gunners and all Specialists but Mutants. This can be used while you're braced with your flamethrower, which makes it a surprisingly synergistic pick that will not interrupt your combat flow much at all.
There's not a lot of requirements to keep here, but critical hits place 2 stacks of Burn on enemies, making the Blazing Piety keystone a good pick. Otherwise you may want to keep Thy Wrath Be Swift in order to continuously use your flamethrower without any interruptions. Fury of the Faithful is the most synergistic combat ability, which allows you to rapidly approach enemies to use your flamethrower or to deal burst damage with your melee weapon.
The Flamer is the primary focus of this build, obviously. Unbraced left clicks have extremely good stagger, and can be even better if you wish to take Fan the Flames so you can keep control of enemies. You can stagger Specialists repeatedly with left click and keep them in place in order to instantly kill them with a throwing knife.
I choose the Antax Combat Axe for its general usage and good light attack patterns. I recommend having a weapon that can deal with anti-armor to compliment the build.