So first, for the floors before Vanaduke you'll not anything else than your standard elemental sword. A piercing one can be used for trojans too (there are some silkwings/oilers/wolvers, but so few it's not worth gearing specifically for them).

So your problem will be for Vanaduke, as, during phase 3 and 5, you can't easily and effectively use a sword (to do that, you would need either watering all the fireballs which require an enormous amount of teamwork, or use only charges, which is sub-par).

So you need a gun, preferably a piercing one. First, Callahan is a bad choice. It has a low dps and his meant to be used against dodgers for pure gunslingers. Save the few wolvers, nothing dodge down there, so you are left with a low dps gun. Instead, you have 3 choices :
- Blitz Needle (or another autogun line, but this one is better for FSC in my opinion). It have the highest single target dps of the game for piercing, but is hard to use, as it roots you in place for a moment each time you fire the charge (you shouldn't use normal shots because they are sub-par)
- Argent Peacemaker. The split damage makes it not that good, but it's actually the 2nd best gun for dps on Vanaduke after autogun line guns. It has a very high mobility and a 6 clip shot. But when the damage will be changed (if it ever is), it will probably fall under the Polaris
- Polaris. It has less dps on Vanaduke than Argent Peacemaker and 3 shot clip, but it's a very good side-arm overall for swordsmen. It'll be better for the levels before Vanaduke since it's normal, have an aoe and can shock (it's also the easiest way to do the puppy room in D25)

If you want a gun only for Vanaduke, I would go for Blitz Needle. It will be a little hard to learn, but after that it'll have massive reward (way faster killing time of Vanaduke, can OS trojans). Note that it's highly recommended to have a Shivermist on Vanaduke, else it's harder (but still doable) to pull off the charge without being hit.

If you want a side-arm you'll be carrying outside of FSC and use a lot, go for Polaris.

As for mask phase : doing a sword specifically for it would be a bit of a stretch. Your usual elemental/piercing sword is enough to take care of it. If you really want to make a normal sword (which WILL land better results, but not THAT much better), both DVS and Leviathan are good choices. I didn't do maths on how much damage you can do for each one, so I won't advise one over the other for the mask. But Leviathan charge makes it useful as an only sword if you ever decide to switch to gunner or bomber.