I run Callahan-Umbra-Storm and the only time you can't attack with a gun with that setup is against mecha knights in arenas (or other areas). That's where my Shivermist really works great, since everyone loves immobile bots so they can charge up their attack and hit from behind. A Leviathan wouldn't hurt to have though.
Personally, if I was using my Hail driver I usually bring a Venom Veiler since the statuses can co-exist, and poison will always be useful (unless in a poison stratum of course) but since you're already on your way to Shivermist glory, you might as well just work on that for now.
Just be careful not to trap a teammate inbetween lumbers or a pack of jellies.

Shivermists ARE awesome, but you should really swop the Cryo driver out for something else. Shivermist does it better. Seems silly to have both. Why not a Magma or Storm driver?

Tairy, most of the teams I play with hate mechaknights being frozen willy-nilly, because they still do their multiple swing charge attack, and it interferes with the primary strategy of "kiting them around in a circle". That technique may come back to hammer you in T3.

Duplicated post.

Hello, dood. There will probably be some TF2 guys jumping into Spiral Knights with the new Steam connection. Be sure to give them regards.

I CAN RECOMMEND:

-The Nitronome's explosion and the Leviathan Blade's charged attack have excellent knockback. Normal damage. Up to three hits if the charged attack is launched point-blank at an immobile or cornered target. (I don't think I need to tell you this, but the Cold Iron line left me disappointed. Leviathan is preferable.) Merrily, both are available right from Kozma.
-The Spine Cone has the same attack power as the Blast Bomb, but its detonation timer gets slower and slower as it upgrades. It does not have any knockback (great for kiting, bad for breaking through a crowd). Piercing damage.
-The Snarble Barb behaves exactly like a Flourish but its charged attack sprays thorn projectiles in front of you. Upgraded forms give more thorns. Piercing damage means its normal attack absolutely wrecks Beasts and Fiends, which would normally love to dodge your gunfire. (Don't worry about the Royal Jelly, his Piercing resistance is almost negligible. You can smash him with this-- but you'll need a different weapon to get through his minions on the previous stages.)
-The Sealed Sword series has the Troika's knockback, but slightly faster speed. (Two hits, most people just shield-cancel out of the first swing.) The Divine Avenger's got a fantastic charged attack (Pewpew multiple gigantic knockback multi-hit swordbeams!) whereas the Faust has a fantastic Curse effect (but you are heavily discouraged from using its charged attack, as it can Curse you too, plus takes forever to charge). Don't mind their speed tooltips, they have the same (slow) attack speed. (The Faust's speed bar was copied from the Proto Sword/Calibur's speed bar; the Avenger's was copied from the Hatchet/Troika.)

ALSO CONSIDER:
-The Brandish series. Just... not the Cautery Sword, it's pretty terrible. (Note that the attached status effects are either rare or non-existent for normal attacks-- they're on charged shots only.) The charged attack generates a random number of shockwaves-- up to three bursts on the 5* versions.

I DO NOT RECOMMEND:

-The Flamberge and Rigadoon are frequently regarded as being inferior to the Flourish and Snarble Barb because their attack power is much lower but the added status effect is applied too infrequently to make up for it. Not terrible by any means, but the tradeoff is often perceived as not worthwhile.
-The Cutter series scores an absurdly high number of hits, but leaves you very, very open to retaliation. Ironically, the fastest of swords leaves you with more exposed time than the slowest of swords. The charged attack exacerbates this issue-- you can perfectly slip behind an unaware Gun Puppy and release a charged attack... but before you finish your maniacal sword frenzy, it has already turned all the way around and shot you in the face point blank. It has its uses, but doesn't sound like the thing you're looking for.
-The Spur --> Winmillion would be awesome if it didn't trigger dodge/block reactions with every swing. Now you have to CHASE them as they dodge-dance away from you? No thanks.
-The Crystal Bomb series can score great damage if you get multiple shards to hit the same target. The problem is, they're projectiles and trigger dodge-reactions in the stuff that's weak against it. So never mind. (The Ionized Salt Bomb is the only bomb with a Shock effect outside the Starter or Party Pack bonus items-- but it takes forever to charge.) Also, no 5* upgrade.
-Ash of Agni is fantastic anywhere that doesn't involve fire-based monsters, but our mutual (ragecrafting) friend is already working towards this. (I guess you can be Rabarta, World of Ragecraft can be Rafoie and if I can get a Voltech with a good UV, I'll be Razonde.)