This was originally going to be a build post, but I am still tinkering with different concepts to see what does the most damage.
In the meantime, I do have a couple of general tips that I wanted to pass along:
Short Fuse: Facepuncher shots do not proc Short Fuse.
Scorching RPMS: Melee attacks, including Facepuncher pellets, do not crit, but White Elephant stickies stuck to crit spots can. These stickies can benefit from SRPMs’ bonus crit damage, but given that you only have a 30% chance to lay stickies, I would argue that this skill is not worth the investment.
Pull the Holy Pin: PTHP does not enable the Fish Slap to crit.
The obvious choices here are going to be teh Bloodletter and Mind Sweeper. Skill rolls don’t matter a tremendous amount on either, but if you use a White Elephant artifact, you will want +2 TRL on the Bloodletter in order to reach 1 HP.
As for state rolls, melee damage and splash damage are obvious choices, but the Facepuncher and Fish Slap do not benefit from weapon type damage bonuses. Interestingly enough – the stickies left by the Facepuncher do appear to benefit from Hyperion crit and damage bonuses.
Your two best options for artifacts are going to be Knife Drain White Elephant (for survivability) and elemental Stone White Elephant (for damage). The best stat rolls here should be obvious – melee damage, AOE damage, and mag size – again the Facepuncher and Fish Slap cannot benefit from weapon type damage bonuses, so don’t bother with them. Bonus elemental damage and DoT chance can be good on Stone artifacts if the roll matches the artifact’s damage type (radiation on a radiation stone; corrosive on a corrosive stone, etc).
The Stinger’s melee novas benefit from Mayhem scaling, so this shield is a really good option, particularly if you are using a Knife Drain artifact. The novas also have a chance to place sticky bombs that hit like a truck.
You can also do a neat trick with the One Shotter shield and Knife Drain artifact so that you fire off an amp’d shot every 1-2 shots. The net damage bonus from this tactic is pretty decent and comparable to the bonus you receive from the Stinger.
The Old God’s elemental damage bonus does apply to the damage dealt by elemental Stone artifacts, so that is another option.
Not really to much to say here except go for the Redundant variant as it will lay down more sticky bombs per trigger pull. The three best anoints for the Facepuncher are 300v2 (for mobbing), consecutive hits (for bossing), and uRad (option for 1HP builds). While element NTM anoints work on the Facepuncher, I found that its low ammo count meant that I was reloading more often than I would like. The bonus melee damage anoint also works, but its duration is so short that it is not really worth the trouble of jumping in and out for IB for it.
The Fish Slap works well on the Knife Drain build, though you probably won’t hit those triple digit million damage numbers that you see on other builds because your Facepuncher will be constantly resetting the Groundbreaker perk. That said, it benefits from Moze’s splash damage skills and still does single digit millions of damage on Mayhem 10, so they definitely worth using.
Other than that, just use a CMT and spec into PHTP and Vampyr.