Combuster is the best choice by far I have tried all the other brandishes/DA and combuster is my go to weapon... Only other one that comes close is voltedge for LD.

More of a preference thing, Brandish and Sealed Sword weapons are perfect side grades. Specifically speaking about these two weapons in FSC...

Divine Avenger:

+ Strong single hit normal attacks that hit multiple enemies.
+ Strong knockback.
+ Wide fanning charge attack that hits multiple times.
- Slow speed and hard to use without ASI.
- Significantly lower DPS vs. Brandish.

Glacius:

+ Strong combo attacks.
+ Medium speed and easy to use without much ASI.
+ Straight lined charge attack that hits multiple times.
+ Significantly higher DPS vs. Sealed Sword.
+ Freezes. Can save your life in some situations. Allows set up for more charge attacks.
- Freezes. Can seal your fate in some situations. Reduces damage dealt if enemies freeze mid charge.

In my opinion, Glacius would be more useful. While DA is a great defensive sword, it's hard to use vs. Zombies since they love to distance themselves before attacking; using the combo is very dangerous seeing as there is so many mobs. Glacius will do a good job of providing high DPS as well as safety if you use the charge attack to freeze enemies, but keep in mind that freeze can help just as much as it can harm if you. Another thing you should know is that Glacius' usefulness diminishes as you get more skilled, so consider investing in a Combuster or Voltedge.

Voltedge is not unambiguously stronger than Combuster in FSC. Which you prefer depends on your playing style. For me and many other players who have commented on these forums, Combuster is preferable in FSC. We've had this argument many, many times. Here's the brief version:

* Regular attacks: Tie, because they are identical.

* Charge attack immediate damage: Combuster wins, because shock often prevents monsters from riding the whole charge (like freeze).

* Knockback: Combuster wins, because its charge knockback is predictable, unlike Voltedge's.

* Status damage: Voltedge wins, because almost all monsters in FSC resist fire.

* Other aspects of status: Which one wins depends on your playing style. Shock can be really useful, but like freeze it also hinders your kiting, shield-bumping, and knockback.

Well, adding on to my point of Combuster and Voltedge--

Voltedge is generally, in my opinion, better. It's safer, and in large crows (specifically the ones in FSC, which spawns tons of Slag Walkers at one time), the shock damage that it does can end up doing more damage than the blade itself, due to chaining shock causing everything to take damage. The only reason I prefer my Voltedge to Glacius or Combuster is because it freezes enemies in place, allowing me to safely charge up another attack, and unlike Glacius, they still take a bit of damage while I'm doing it. Another thing Voltedge has over Glacius is the fact that, although they both freeze enemies, Voltedge can still push back enemies to get them out of your face when you use the charge, keeping you significantly safer than a Glacius charge.

However, if you have access to Charge Time Reduction (ex. Trinkets, Sprite Perks, UVs, or the Chaos set), then Combuster is astronomically better in terms of safety. The fact that it has constant push-back makes it easily the safest Elemental Brandish, but this is only if you have enough Charge Time Reduction to keep up a constant barrage of charge attacks. Otherwise, enemies might be able to get up in your face before you can set off another charge. Voltedge sort of prevents this, especially since it can prevent enemy movement while they're attacking.

So, to make it a little more simple:

Combuster with CTR > Voltedge > Combuster without CTR > Glacius

I'm not even going to mention Divine Avenger because, let's face it, compared to the monster that is a Brandish, Divine Avenger is a gimmick weapon for FSC. Now, if you were going to make a weapon to use just as a general Elemental weapon, not just for FSC (as proposed in the title), Divine Avenger is a good and arguably more fun choice, although a Brandish fits better for general use.

I routinely gather up crowds of zombies in the middle of depth 25 using Combuster. Predictable knockback makes this easier, not harder. The zombie AI is so slow that you don't need shock to stick them in place. Just maneuver so that they don't launch an attack, and they'll be paralyzed with indecision.

Actually, there is one problem in bunching zombies with Combuster: They tend to die before you can build a big bunch.

Okay, that last bit was probably too snarky. Voltedge and Combuster are both extremely viable in FSC. Differences in skill, planning, lag, party size, etc. dwarf the slight differences between Voltedge and Combuster.