Guns: hmmm. Yeah, could work. One thing though: sorry but no valiance. As a gunslinger it is your job to specialize in damage types. Normal damage is a nono. For your second weapon slot, you should get a Polaris. They are an awesome gun, with knock-back, elemetal damage, and the chance to shock! They require a bit of practice though, because unless you are solo, you can't just spam the bullets. This will cause monsters to be knocked everywhere, and people will hate you. You need to be able to use the bullets to shape and corner mobs.

Shield: good choice. Elemental is very common, fire is one of the worst status effects and is commonly found in the Fire Storm Citadel, and shock is also one of the worst status effects.

Armor: don't go with justifier. The piercing defense it provides is no match for the elemental defense of the nameless set. Your toxic needle will not he helped by the ASI, but your Polaris sure will!

Current set up: No. What I suggest is get your owlite shield, and gear up for the ironclaw munitions factory. You're going to need to run it 5 times for a pulsar. Good IMF gear that is easy to obtain is a magic cloak and hood. A good gun for the IMF would be a firotech/cryotech/prismatech alchemer. (note: if you absolutely fall in love with your alchemer, and after using a pulsar decide that it's not that great, then stick with your alchemer). Good swords would be the wrench wand, fireburst/iceburst brandish, a searing edge (not that recommended), or any thing without a lot of knockback (you don't want to supercharged quicksilvers by pushing them onto shock grates).

the Pulsar shots will actually explode, which deals damage to multiple enemies, so can destroy crowds way quicker than with a single target alchemer. The explosion provides knockback which can be used to shape crowds. As for the nameless set, Here are the monsters that do elemental.

Exploding blocks
Retrodes
Lumbers
Elemental mecha knights
Gun puppies
Charged quicksilvers
Element grates (commonly, but not only found in the IMF)
Shadow fire (found in the firestorm citadel)
Gremlin scorchers

Many of those monsters are very annoying. As for monsters that do piercing:

Wolvers
Slimes
Chromalisks
Floorspikes

None of those pose a very big threat other than Alpha wolvers. Ask some experts and even they will agree that piercing defense is waaaay less important than elemental.

Yep. Polyps are slimes

Yeah, you're right. People do tend to forget polyps when they think of slimes