Harlowe Instant Melts Endgame

Item locations posted at the bottom

Guns:
Mobbing: Katagawa's Revenge is simply the best option, especially if using the Scientist class mod. Try to find one with both Jakobs and CoV part. Jakobs will give you much more boss dps, while CoV gives you more ammo economy.

You can also use a Fast Firing Vladof SMGs, preferably with Double Barrel and Ricochet, you want it to be elemental as well so you can take advantage of the Skeptic class mod, Lucian's Flank works too it just feels like crap

Bossing: Luty Madlad with the Daedalus shotgun underbarrel, Convergence, Katagawa's Revenge all work great

Repkit: Can use a legendary Shiny War Paint as well for movement speed/rate of fire. For bossing you could use one with enrage + elemental affinity or something like that but you honestly can use anything. I think Triple Bypass could be BiS depending on the rolls but I don't have a good one

Class Mod:
All rounder and my preferred option: Skeptic with preferably +5 into Indiscrete Math or Bright Future is fine too. This will work regardless of your skill tree.

Scientist is another great option, but it forces you into speccing Confirmative Reality, it's great for when you're in a group though. Try to get +5 into Periodic Table Flip and +1 into Ripple Effect, look for splash damage, action skill damage, movement speed, damage reduction

Bossing: Atomic Driver is best but the extra damage is honestly unnecessary - try to get as many points into Field-Ready Prototype if possible. Try to get action skill damage + anything else as the secondary stat maybe skill damage if that's possible or any defensive stat that looks appealing to you

Shield:

My preferred shield is Watts 4 Dinner
Firewerks is good for mobbing but kinda unnecessary IMO
Protean Cell is good for bossing w/ the 50% dmg buff
Watts 4 Dinner is a good all arounder if it rolls with 'Siphon'
Super Soldier is great for movement speed, the ammo regen is a little weak but it does SOMETHING
Pandoran Momento is crazy for defense

Enhancement:
When using Katagawa's Revenge that has a Jakobs part you want a Bounce Pass enhancement for +1 additional ricochets which will drastically increase your boss DPS output, try to get it paired with the 40% chance to ricochet on NON crits if you don't have a penetrator augment throwing knife
When using Luty Madlad you want a Daedalus enhancement with 50% fire rate stacking.
When using Convergence idk the fire rate ripper one or something
Stats: Mag Size, Sniper Mag Size SMG Mag Size, Fire Rate, SMG Fire Rate, Splash damage, Gun damage,

Firmware: Generically best setup would be:
3 High Caliber 2 GOOJFC

For Bossing 3pc High Caliber seems to be the best due to the pierce allowing Katagawa's Revenge to hit more times per second, if using a different gun then 3pc Deadeye is your best bet

Ordnance: Penetrator Augment knife OP as always

Guns:
Katagawa's Revenge = Driller Hole, Unpaid Tab, Queen's Cradle
Luty Madlad = Mimicron, Order Bunker, Tipping Point
Lucian's Flank = Saddleback, Ripper Drill Site, The Old Scrape
Convergence = Bramblesong, Auger Mine, Stoneblood Forest

Class Mod:
Scientist = Radix, Arch of Radix, Carcadia Burn
Skeptic = Queen Callis, Her Flaming Vision, Moon-Called Throne
Atomic Driver = Origo, Terminus Range, Arch of Origo

Shields:
Firewerks = Splashzone, Coastal Bonescape, Fadefields
Protean Cell = Fractis, Order Bunker, The Crawlcoft
Watts 4 Dinner = Origo, Terminus Range, Arch of Origo
Super Soldier = Vile Lictor, His Vile Sanctum, Umbral Foundry
Pandoran Momento = world drop