Backing up on @Thinslayer here, the Brandish lines are some of the best go to Elemental weapons for FSC. There are other options such as Divine Avenger, but it's outclased in terms of damage output.

Divine Avenger is more for the playstyle wanting to be safe or easily interrupting, with it's wide swing arc and large knockback against groups of enemies. The second swing can flinch targets, which can be ideal in some situations. The charge attack covers alot of ground, but suffers from increased charge time (But less than Gran Faust's), having a long windup after release, and due to a oversight the beams spawn behind you first and thus can get eaten by walls while in corners.

As for the Brandishes, all have their pros and cons, let's go over them:

The Combuster: Is considered literally the DPS buster weapon, especially for solos. Easily outclassing it's other elemental cousins in pure damage, on top of Fire status infliction. This is due to the charge attack can carry targets with it without fail, unlike Freeze and Shock on the other two.
However, in a place like FSC, Combuster loses access to Fire infliction on most of the enemies present and thus some of it's damage output, most notably on the Slag Walkers. Fire can also alight some of the few Oilers present, potentially making a mess of fire puddles everywhere, on top of granting them resistance to Elemental damage.
There is one exception, the Almirian Royal Guards summoned by Lord Vanaduke are not immune to fire despite their appearance, and thus can be inflicted with it. However most would opt to crowd control them instead, due to their high HP, attackable only from the back, and a tendency to charge all over the place causing havoc.
The charge attack also requires some careful care, mindlessly spamming it can cause one to end up knocking enemies right into teammates, potentially getting them cornered, hit or even killed.

The Glacius: Is considered the weakest of the trio, though still has it's uses. It's Freeze can act as a potential crowd control weapon and keep hordes under control and stacked without the use of weapons like Shivermist Buster, even from a safe distance without having to focus connecting the direct hit on charge. Also allows more optimal setups, such as Freezing a Trojan and then Blitzing it's back with no danger.
Also can thaw the Oilers if they have been lit on fire, undoing their resistance to Elemental damage as a result. This rarely comes into use, but it may help in some cases.
However, Freeze is removed in one hit from any damage source unless instantly reapplied, making it hard to rely on in some situations. Freeze can also trigger on the direct hit via charge, which can get one hit at close range while an enemy is going for a swing and are too strong to be interrupted in one hit, especially notable in larger parties due to bigger health pools, thus more flinch resistance.
Freeze also screws up the usage of Vortexes, as Freeze cancels all movement and prevent enemies from being stacked by Vortexes, unless after they been dragged in first. Not recommend if you have a Vortex yourself, or a user in the party without careful use with the two together.

The Voltedge: My personal choice and signature weapon, lies between outdone by Combuster to being the king of balance in damage and crowd control. The Shock infliction can be reliable when used properly, and benefits from increased DPS on stacked targets, due to Shock inflicting small bits of damage to any other enemy close to one inflicted with Shock during it's duration. Combine this with inflicting an entire stacked group with Shock, the numbers start rolling in very quickly.
Due to this benefit, it can be considered the best pairing with Vortexes, allowing taking easy advantage of this status mechanic. Shock has a added benefit of temporary making enemies immune to knockback when it ticks, which can be both advantageous and disadvantageous. This also can interrupt enemy attacks on it's own, but only when an enemy is about to wind up, not during attack animations.
However, both of these can be ones undoing. For one it can make enemies unpredictable, especially Trojans and Slag Walkers. Like Freeze, it can cancel pushing an enemy with the charge attack, getting one hit at close range. Though i'll say it's not designed for this purpose in the first place, unless you have ridiculous luck in Shock not triggering from the charge attack, which is quite rare.

An note that both Combuster and Glacius can be crafted normally and their recipes found at Basil, in some rank mission lobbies, or in the Hall of Heroes. Voltedge's recipes are only obtainable from Sullivan, the PvP merchant. It's entire set requires a total of 300 Krogmo coins, forcing one to play Lockdown/Blast Network, or buy it directly from someone else via unbinding from Vise.

All 3 though have potential uses outside of Firestorm Citadel too, personally i am a fan of using Combuster against Constructs, and Voltedge against the Undead. The latter completely annihilates Kats unless they are their Shock variant.
Glacius i currently only use in Shadow Lair: Ironclaw Munitions/Darkfang Munitions Factory unless i opt to switch to it to counter Shock-themed Undead over Combuster, though rarely i do this. Same goes for Fire Constructs, instead i'll always default to Voltedge in that case unless Quicksilvers are in bound.

Hope this helps in your decision.