Blackhawk has serious problems hitting Wolvers and Gremlins and does poor damage to Slimes, though. You'd be able to hit Wolvers and Gremlins much more consistently with a sword. They don't autododge swords as often as they do bullets.

...Anyway...

Specific-purpose:
Flourish series (I like the Flamberge line but Final Flourish is pretty good too) versus Beasts and Fiends. It destroys a lot of the monsters you probably hate: Wolvers, Devilites, Silkwings, and Trojans. It still does normal damage versus Gremlins and Undead, too. The charged attack is three rapid swings, and you can land all three on the same target without needing to immobilize or corner the enemy first (like the Calibur needs to).
Faust series can wreck Slimes with its shadow damage and its good knockback leaves you less open for retaliation (Just swing once and shield-cancel). It'll also put a hurting on Gremlin Menders with its shadow damage (again) and chance to curse.
Cryotech Alchemer series, once at 3* and up, is handy against Constructs and Undead. Gun Puppies can't turn towards you while frozen, and Freeze is one of the TWO status afflictions that work on Phantoms. (The other one is Stun.)

Other useful general-purpose weapons:
Calibur series. Preferably the Leviathan line. I got the Carver and regretted it when I found out the bars lie. :(
Shivermist Buster. Especially with a party. Plant it, freeze the enemy, and your allies can crush them with charged attacks. Also allows you to stall a group of Phantoms nigh-indefinitely while your team runs ahead to clear the zombies out for good loot. (The only problem is you can't re-freeze them once they're knocked out, so you have to hope you can catch them in another ice-cloud when they wake up.)
Ash of Agni. Especially while solo. Plant it, lure them in, and while they're burning you KITE LIKE A MAN. Though be careful around Oilers and, of course, Fire-themed stages. (Limited use in Citadel due to the vast number of fire-based monsters in there.)

I don't see many people with the Venom Veiler. It'd probably be really useful in Danger Rooms and Arenas though, considering you'll always see healers plus it's always nice to lower the enemy's attack power and defenses.