Ash of Agni seems like the obvious choice. It doesn't usually upset parties and it has great DPS against an entire room of monsters. Granted, fire's not useful in the citadel, but for the rest of Tier 3 it should be pretty sweet.

For when you can't use fire, I'd go nitronome (if you typically run solo or with friends who don't mind adjusting their strategy to accommodate the blast) or pick your favorite gun. I'm not too familiar with many guns (I've used blaster line and cryotech line, but you have ice covered with shivermist), so ask more of an expert if you want to go that route.

@Sick - With 4 weapons, the gun doesn't have to be used with the bombs; it can be held in reserve until a situation is encountered that makes bombing impractical compared to a gun. So the fact that bullets free a monster from the ice of Shivermist isn't necessarily a problem. The leviathan also frees frozen monsters, but you didn't advise him to ditch the sword. The key is to use the right weapon at the right time.

Also, I fail to see how a gun slows down anything with the Veiler. The gun damage is boosted by the poison, and if he uses a Driver he can add a second status ailment. Poison with shock/fire can end monsters pretty quickly and that's without factoring in bullet damage.

Elemental or Shadow Damage Gun; So probably alchemer series or maybe Peacemaker if you want to start investing towards Vanaduke Items.

Its an auxiliary weapon it doesn't need to synergize at 100% with everything else, its there to give you more options in a fight.