I see no point of getting antigua if you already have kilowatt pulsar, heavy pulsar, needle shot and shadow alchemer. The four guns you have are really all the guns you will ever need, perhaps save an antigua as a 3* piercing for the sake of beasts and fiends, they are really nasty as your weapon combos are really not good for fast enemies. You can use the excess tokens and get more antiguas and sell them to the NPC for some quick cash. Gunner update is a light year away, so you might want to get use to the needle shot agaist the fiend, as for beast, the bane of your current arsenal, maybe you can get a magnus or the 3* antigua to make up for it.

Judging by your rank, you should have already beaten RT now, you know who the last boss is, so for the last boss you will need a gigawatt pulsar and a strike needle so focus on the two gun as your priority, keep both guns at 4* as there is no point of getting 5* thanks to the radiant drought. As for shield, there are currently no gunner specific shield other than SSB, the one you currently have. If you want to focus your fund on the gun upgrades, then you should make an owlite shield, good for the end game and not so shabby else where.

My opinion is that the most powerful guns, which you should get first, are:
* Blitz Needle (massive piercing damage)
* Storm Driver (high elemental damage in expert hands, useful status; Nova and Magma are also good)
* Umbra Driver (high shadow damage in expert hands)
Honorable mention goes to Polaris --- not for damage, but for crowd control.

If you're going to call yourself a gunner, then you should sample all of the gun types. So you should also get something like Neutralizer and Callahan. Don't bother with the Antigua-line guns; they are neither powerful nor interesting.

You need a 5-star shield to clear the Hall of Heroes at some point. It might as well be Grey Owlite or Crest of Almire. Some people like Omega Shell now. But in the end you might return to Swiftstrike Buckler, for the ASI, even though one hit from a monster will tear it up.

Numerically speaking the Sentenza and Argent peacemaker dole out more damage to neutral enemies than a Valiance.

The 6 shots while weaker individually do more every 2 hits than a valiance's one hit.

The downside is that you have to click more often which can put a strain on your fingers.

Most gun charges allow you to be fairly close to an enemy, for the Antigua charges to be good you have to fire before they walk into range (best with terribly slow enemies or ones that can't reach or shoot at you).

Being temporarily immobile is a pain but guns like the blitz needle show that you can be rewarded for your risk.

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The biggest threat to a gunner is ranged attacks as most gunners will stay out of melee range, there are few piercing ranged attacks (polyps and some lichens, chromas are a ranged joke), elemental and shadow type monsters have more of them. (Tortodrones also have a ranged piercing attack but they are event monsters)

Shadow: Kats, howlitzers, greavers, devilites, bombies, phantoms.
Elemental: Puppies, retrodes, mortafires, vanaduke, roarmulus, battlepods, scorchers, thwackers, stalkers.