Mmmm I like switching up my FSC load outs but here are the two main ones I use the most.

(Cookie Cutter)
Slot 1 Combuster
Slot 2 Electron Vortex / Haze Bomb / Acheron
Slot 3 Argent Peacemaker
Slot 4 Blitz

Combuster would be my primary weapon and I would throw a couple or AP shots to close distance for more DPS.

(Survival)
Slot 1 Combuster
Slot 2 Storm Driver
Slot 3 Hail Driver
Slot 4 Blitz

Combuster and my Drivers are my Primary. I use 4 slots and double switch my drivers to avoid taking DMG since its range attacks with status applied for immobility and high DMG with shock and ricochets. And Combuster could just be used traditionally as well for some quick DMG or pushing enemies away or grouping to get best DPS out of your drivers. Switching almost perfectly takes a lot of practice and endurance if you are trying to switch two guns with 2 additional tools in the way… I do it because it’s fun and looks fun. It’s rather effective and useful.

There are a number of guns that perform about as well as the mixmaster, but no gun is just flat out better than the mixmaster.

For starters, mixmaster shoots six storm driver shots with each combo and doesn't require switch shooting to perform this maneuver. The boomerangs make it easier to land shots, especially against things like trojans, but come at the cost of ranged damage. This makes it an extremely potent mid-range weapon, almost like a sword. Its charge attack also does storm driver damage, but lacks the massive number of bouncing projectiles to make it as incredible and instead is more useful for just landing shots on enemies that dodge like kats and devilites. Mixmaster is definitely one of the better handguns in the game.

The only reason the drivers aren't just flat out outclassed by mixmaster's 6 shot combo is because of their charge attack, which does a huge number. Switch shooting incredibly well would also outclass mixmaster's shenanigans, but would require the user to make room for two weapons and play incredibly well with them. Nor would switch shooting be that huge of a difference- if you always double hit, you'd have to fire at least more than 3 times per 1 mixmaster combo to outdamage the thing. Lastly, the drivers do have superior range, which is moderately useful but less a deciding factor for levels like firestorm citadel that feature loads of highish health monsters in somewhat tight areas. If you want to get one, the drivers are certainly viable but definitely not a straight upgrade.

Arcana would be a DPS decrease overall, but sports higher range and a bit of knockback. This makes Arcana a best fit for playing defensively from a distance and therefore a better option on loadouts that depend highly on being able to survive in a time of need. But you probably don't care for that sort of thing and would rather just have the DPS most of the time.

Argent peacemaker is better at breaking blocks and hitting exact areas, like a lazer pointer. You shouldn't be considering it for firestorm citadel unless you're getting it for something else and don't want to craft another weapon. Control over the environment is relatively worthless in FSC.

If anything, I'd look into getting and mastering the elemental omega tortofist. There are TONS of clumped up enemies in firestorm citadel, which means that with practice you can get your DPS to go through the roof. A good charge on the tortofist can hit 3-4 times on as many slags as you can lodge the projectiles between, and that sort of multiplicative damage adds up incredibly quickly.

The only enemies that I have a lot of trouble hitting for an absolutely amazing amount of damage are wolvers in T3, turrets that never move, and the more intelligent enemies with shields like tortodrones and deadnauts. A lot of enemies require a lot of prediction and creativity to regularly land hits- like devilites (aim where you think they'll dodge, and you can often hit 4-5 devilites) and kats (aggro all + aim at walls next to you as they come at you). But once you figure them out, they're usually pretty easy to land hits on. It also doesn't handle the huge griever waves very well.

The ones that are incredibly easy to murder in huge numbers are the ones that move about and chase you at a slow to moderate pace. Basically, mecha knights (which humorously try to shield and then walk into it anyway), retrodes, most slimes, lumbers, all of the minis and of course zombies.

Larger mobs like giant lichen colonies, Vanaduke, the jelly king and tortodrones also have a hard time with them, as they get stuck on the walls it throws up. They try to come towards you and aren't really used to pathfinding so much.. which results in them getting pressed into the damaging wall and getting hit by the next barrage that falls, repeating the cycle. It's less damage than using an autogun line (since it's just one target), but quite a bit safer as well. Vanaduke not so much as his main attack is the smash, though it does turn his dash against him and does do enough damage to let you occasionally not bring blitz needle if you don't mind a longer fight.

Hm, so elemental would be good for 2 of the 3 famlies that it would be effective on, and neutral on the slimes... I think I need to work on a good tortodrone loadout for the next event, time to search the forums. :D

Hm, Traev does make a good point, AP used for finishing off zombs/pulling aggro would work well, and since it fires quickly on its own and has the undead bonus, you can go with sword bonuses instead and the gun is still fine. Of course, also makes it a poorer weapon for a dedicated gunner, but as a third weapon for a swordie with brandish and blitz, seems like a better choice than I was thinking at first.