I've already done FSC a zillion times and even helped carry newbs through it, but I still can't decide on the best loadout for it. So I'd like some advice. I have lots of info to help you help me.
My skills are the following:
I am average at dodging things. I'm good enough to have survived shadow lairs with less-than-stellar parties, but I'm still prone to stupid errors in judgement more than I should be. I'm a fair shot with a gun, but have very little experience with alchemers and internal ricochets. I'm not a bomber, but I have decent timing skills should I need to learn the art. My main strength is swords, and am proficient in medium and slow swords, but not so much in fast ones like strikers or spurs.
I presently have only two weapon slots and no trinket slots, but I can afford more if need be. I would prefer to plan a loadout without expanded slots. If I absolutely need another weapon slot, I can get it.
Vanaduke's bullet hells tend to give me the most trouble, followed closely by his 5th stage, where, if things go awry, I often get blindsided by slag guards and smashed with Vana's mace when he's offscreen. If things go right, I can usually dodge slags if I'm paying attention to them. Trojans and zombies don't give me trouble except in closed spaces; then the Trojan buffs and zombie lunges are my worst enemies. You all know how often there are close quarters fights in FSC. I almost never hit shadow fire or spikes, and almost always dodge red rovers.
Now for the relevant arsenal. I only carry 5* gear into FSC, except when leveling. I'm fanatical about leveling things via FSC, so unleveled stuff takes precedence.
Armor sets:
Fallen lvl10, Vog lvl10, Heavenly Iron unlvld w/ UV fire med & curse med (net), Skolver unlvld, Ancient Plate lvl10
Shields:
GOS lvl10 UV pierce low, CoA lvl10, VPS lvl10 UV elemental low, Ancient Plate unlvld, BTS lvl10, SSB lvl10
Swords:
FoV unlvld UV ASI med, DA lvl10 UV CTR low, Glacius unlvld, CIV lvl10 UV undead high, Levi lvl10 UV ASI med, FF lvl10 UV ASI low, BTB lvl10 UV CTR low & Undead low (if I remember right)
Guns:
Valiance lvl10 UV CTR low, AP lvl10, Blitz lvl10 UV CTR low, Polaris lvl10
Bombs:
Shiver lvl10, Spike Shower unlvld (4*, I know, but I made an exception here because I'm aware of bomber preferences for it)
This isn't my entire arsenal, but only that I deem relevant to FSC. Any opinions on how I should equip myself, or any skills I really need to perfect or know about?
Well, personally my gear of choice is the Deadshot Set with Crest of Almire and Voltedge + AP as my weapons. Sometimes Needle + AP. I don't use trinkets or any other fire resistance. I carry Remedy Capsules in case I get burned, which is typically uncommon.
Armors:
I wouldn't think Vog is the best choice. It provides defenses that are only used by Red Rovers (And the missile puppy), which you have said you don't have trouble with. The only reason I'd use Vog is for the fire resistance, which you said you don't get very often either.
I would only go with Skolver if you use swords almost exclusively, shield effectively (or don't get hit by zombie lunges), and don't plan on attacking the Boss himself. If you don't get hit much, Skolver with an elemental sword would be good for killing Slags at the end and throughout FSC.
The Fallen set is a pretty good choice if you already have it. It is very limited in its field of effectiveness though. It works well for FSC but not SLs and is not versatile enough to be used when venturing randomly through the Clockworks. But for FSC itself, there is little wrong with it. I would use this armor out of the ones you listed.
Heavenly armor is a good go-between from Fallen and Skolver. It gives two low buffs, but trades fire resistance for it. If you plan on killing trojans with FFs or BTBs as well as using an Elemental sword for the rest of the enemies, I'd use it.
Ancient Plate is...not recommended in my opinion. It provides lots of normal defense and health, which makes it rigid for clockwork-use, but for FSC you're likely looking for specialized use, like the Fallen set. The debuffs on it (though movement is extremely minor) aren't great either. I've never actually used it, but I would only use it if I didn't know what I was facing.
My favorite shield for FSC is CoA, because of the combination of great defenses and fire resistance. It has saved me from the Vanaduke-Mace-Shadowfire-Combo before. The fire resistance and lack of elemental defense can contradict each other, so shielding against flame traps and Red Rovers is not so effective. If you plan on using it for those, you'll probably want some sort of UV or a different shield.
For guns, it depends on your task. AP is by far my favorite sidearm for all things FSC. It does ~170 damage a bullet (When you wear deadshot...not sure otherwise) with a good clip and great mobility. It shoots quickly and precisely, which is great for flipping switches or destroying those wheel-spawners from the front (They are Construct and take extra damage from it). AP is useless for Vanaduke though if you have a Shivermist. It constantly breaks the ice and makes the ice bomb useless.
Valiance, like Ancient Plate, is a versatile weapon but not suggested for FSC because you can specialize for this cause. The only reason to use Valiance is for the Mask, because Vanaduke's Mask resists special damages. The mask generally isn't a problem though. If it is for you, or if you are designated to destroy it, the Valiance is probably a good bet.
Polaris I would only suggest if you have a hard time with zombies or hordes of enemies. Generally speaking, the zombies are sluggish. But if you are overwhelmed easily, the Polaris can help you by pushing them back as well as shocking them, which can disable their lunges. In close encounters though, the Polaris doesn't get enough time to generate a good push-back.
The Blitz is one of the most ubiquitous weapons of FSC because of its Vanaduke-Slaughtering charge attack. I take a needle if I am fighting Vanaduke. Otherwise, I find it a debilitating and highly immobile weapon. Some prefer it as a sidearm because it can OHKO (With a charge attack) a Trojan. I don't have many problems with Trojans, and I'm generally not the one fighting Vana, so I tend not to use it.
Shivermist is generally reserved for Vanaduke as well. If your team is lacking in firepower or skill, it can slow down enemies a great deal. Both this and Blitz will likely be your variable weapons. If someone has a shiver, don't use yours. If nobody does, use it. Similarly treat Blitz. Just don't use Shiver + Blitz.
Finally, swords. Your FoV has a good UV, but I'd say a CTR UV would be better. I've never used one though, so I can't tell you much on FoV. I know the charge can be very deadly to Slag Guardians on Vana, because they can be lit on fire.
DA is alright. I believe it used to be the King of FSC. The charge attack clips behind you easily though, and is useless unless you are in an open area. It does really good damage though, and the 2nd hit has a good knockback. Just be careful you don't throw enemies into your allies like when you use a Polaris though.
I'd use CIV over Levi, despite your ASI UV. With the CIV's UV on top of its innate undead bonus, it can be a zombie killer and a Vana Mask attacker. I wouldn't use it unless you plan on attacking the mask specifically, though, as (even with undead Ultra) Normal damage weapons are nerfed more than others.
I'm not exactly sure why you have both FF and BTB, I'm assuming for the charge attack, though I rarely see either one's charge. There is little to be done with pure-piercing here, I think. It might be average on zombies, and kill the um...6? wolvers in FSC, but it won't do anything to puppies. It is good on Trojans though. I've never used BTB's charge, but FF's charge on a Trojan's back hurts it a great deal. Just be sure if you use it as a trojan killer not to wear the Fallen Set. Still, I wouldn't use one.
Glacius is, in my opinion, not a great Brandish to use here. The charge on Brandishes is massive, but the slags will get frozen by the second explosion. They will miss the other, like, 6 explosions of 200+ damage. Comparing to others, Voltedge can debilitate slags by shocking, but sometimes lets them ride about half the wave. Combuster is, believe it or not, a great choice for FSC. It lights Slag Guardians on fire, and slags ride the entire wave of explosions.
I'm unaware of any particular reason to use DBB in this stage. It's a good bomb, but I don't see why you'd use it in particular here.
Overall I would use: Fallen Set (If you don't plan on killing Trojans, and because of your UVs, Heavenly Iron if you do plan on killing Trojans), CoA, DA and AP. If you want to hit mask a lot, CIV and AP. If there is no other shivermist, Shivermist and AP. If there's no other blitz, DA and Blitz.
On to strategy:
I assume by "bullet hells", you mean the fire that falls from the sky when his mace hits the ground? First off, a good shivermist bomber by your side will prevent him from using his mace very often, if at all. Secondly, only water areas that are completely essential to water. There is a limit to how much fire he can spread on the ground. Let uncommonly walked on areas be covered so that fire doesn't coincidentally land on top of you when he attacks.
If you're attacking Vanaduke solely, the Slag Guardians should be someone else's job. If there's nobody available to kill them, I'd kill them before attacking Vanaduke. Yes, they respawn, but not for some time. You can probably finish the whole stage before they respawn again. If you're focusing on them, they're not terribly difficult to kill. Easier than trojans, I think.
As I said before, if you have a bomber, vana's mace is no problem. But if you don't, I'd advise to never be too far above Vanaduke when you don't know where he's facing. Use the camera to your advantage when possible, it makes dodging the mace both possible and infinitely easier.
If you're engaging the Trojan's, using Heavenly Iron, the buff will do a bit less to you, because it (likely) won't inflict shock. It's easier to not get in close-encounter situations with Trojans rather than get out of them though. They're slow and easy to dodge with mediocre reaction speed (Be cautious if you're lagging), and shouldn't really be facing you & close to you very often.
Slag lunges are probably their only deadly attack. Or, maybe, their only attack that actually hits anyone. Relatively speaking, it's fast and fairly damaging. Using your shield effectively is your best defense against slag lunges in my experience, but it depends on your internet really. Both dodging & shielding is more limited the more you're lagging. Alternatively, you could keep your distance and pew pew from afar.
~Sev