I personally love Antiguas, but the worse is the Raptor/Gilded Griffon line. Antiguas are great at picking off wounded stragglers, chip damage on large targets, and/or pulling aggro. However, Beasts will naturally try to dodge your shots as they aggressively assault you, so you need short high damage bursts at close range. Devilities will dodge your shots like mad, Greavers/Gorgos will rush you, and with Trojans you only get brief windows of opportunity to deal as much damage as possible.

The Silversix/Argent Peacemaker however is probably the best one. If you want a gun to deal with Undead, it's really easy to use and effective with very few bonuses. Arcana fills a similar role, but with knockback and less damage. I don't like it quite as much because the knockback makes the Arcana better at keeping enemies away from you, rather than pulling them closer so you can hit them with a sword.

Alchemers are a bit... complicated. No one can really tell you how best to use them (other than maybe giving detailed instructions on switch-shooting shenanigans). You just gotta try them. Sadly, 2*and 3* versions are VERY poor representatives. 4* is a rather big investment before you get a good idea as to whether you'll like them or not. That said, if you're good/lucky, you can insta-kill 3 full health zombies in a party of 4 with a single Nova Driver charge shot.

Blitz Needle is one of the standard guns that you'll need if you want to grind Firestorm Citadel, the best late-game source of crowns. It's a bit cumbersome to use though, and takes some getting use to. I find the Magnus to be more cathartic and straightforward.

The Raptor/Gilded Griffon is a good gun for peppering enemies and taking advantage of the high flinch rate of most fiends, you have a large clip so you can keep shooting and walking. The Charge of the Gilded Griffon is nearly un-useable since most fiends dodge about, it may only be useful against howlitzers but not that often.

All the Antigua guns have identical damage numbers just with the family names switched around. A Sentenza does as much damage against Gremlins as an Argent Peacemaker against undead, or a Gilded Griffon against fiends. The only weird one is the Obsidian Carbine.

The same can be said about the Arcana, just check the damage numbers for one of the other type damage blasters against monsters weak to that type.

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The Arcana is a rock solid choice and can fill many roles as a gun. It doesn't excel at knockback, clip size, range effectiveness, or damage but it can pull any of these jobs while other guns are more limited.

You may want to look into the Alchemer and Pulsar lines as well, these are also powerful elemental guns that fill the roles or Status and range effectiveness or Knockback and crowd control.

The Tortofist guns work best with High gun CTR and when paired with weapons that can easily control monsters (like the heavy swords). The idea is to Launch a Tortofist charge attack and then use the heavy sword to fling enemies into the spikes before they land, or to push then into the spikes after they land.

I would suggest the Elemental Omega Tortofist since most enemies weak to Elemental are slow and easier to move about. The Shadow one is useful too since it works well on Scorcher Gremlins and nearly all slimes.
The pierce one is only useful against Gorgos and Chromas.

Undead:

-Zombies:

One zombie himselfe is very less of a problem because it is too slow to react if you keep just a bit on distance and shoot. The problematic case are many zombies at once that start to create a wall of monsters and corner you with theit high health.

Here I suggest either shield bumping (rising your shield for a brief secound and lower it again) them away or using weapons that knock them back. If you head into endgame stuff or like the mechanic, then you can also choose a weapon with higher dps but a harder mechanic.
I suggest at
#1 Wildfire for its high knockback ability,
#2 Agent Peacemaker for its high clip size and thus the chance to use your shield bump with less delay when required
#3 Alchemer/Needle - I can't say much to Nova Driver since I didnt aquire it

-Kats:
Fast and agile monsters where I recommend a fast and agile weapon:

#1 Agent peacemaker for making missed shots less be a problem.
#2 Storm Driver since its shock status makes kats interrupt their long-range dash
#3 other elemental weapons

-Howlitzers
#1 Wildfire soince its explosion can 'eat' some bullets the howlitzer shoots and it also kills the howlitzer better in his last suicide attack due to Wildfire's larger bullets.
#2 Agent Peacemaker because you can shoot and be able to rise your shield if necessary.
#3 Alchemers because of its damage if shot correctly, but be careful with the suicide attack if you miss your 2 only shots, you cant rise your shield!
#4 other elemetal guns

Fiends:

-Devility:

Fast and agile annoying creatures that I myselfe have the worst trouble with. I did find fallowing weapons for them:

#1 Blitz Neelde's first shot because devility themselve deal much damage but actually have not much health and shooting with 1st clips while never needing to immobilely reload is all you need. Also the wide arc helps you to counteract the dodging attempts of devilities. Remember that it is horribly dangerous to charge with this weapon since once devility is usually in company and they can quickly launch attacks in you while immobile. Also you should got on distance with blitz. This will allow you to hit a wider arc and devilities on the other side have a harder time to attack you.

#2 Storm Driver's bullets can shock and thus give you valuable ticks time to dodge an attack. I use this weapon to spread shock status to them and shocking them lets them suffer even if my next few bullets will miss. Remember that all status effects will vanish if one devility transforms into another job.

#3 Wildfire's explosions keep devilities cornered and you can defend a gasp with this. In open field it might be useful since the explosions can hit it even if it dodges around.

#4.1 Glided Griffin? I tested it has the advantage to be able to block is nice, but too many bullets will miss here, exspecially if you have a natural delay in spiral knights.

#4.2 Riftlocker? It is the better choice for passages since you can shoot more relieably through blocks while antigua-line-guns may shoot a diagonal block instead of the block you wanted. Vs devilities themselve.... I would not say that it is really the saviour of all hope.

#5 Anything else you may handle better than I did. I also tried the pierce version of the tortoguns here, but it deals a bad job, because 1. their movings make it very hard to prejudge where devilities will stop when the shards come and the devilities are small enough to simply walk between the shards without even reciving damage.

-Greavers:

Starting to chase you until they get damage. Shocking them proved more effectly than I thought - better than freeze.

#1 Storm Driver because it will interrupt a Greaver 3x: with two shots and with shocking status that gets very relieable applied. Also the shock of a neighbour greaver may interrupt other Greavers. I recommend here to release one shot, then shield to never need to reload, because then Greavers could rush you.

#2 Riftlocker as direct damage souce with knockback-function

#3 Wildfire to backup team mates or to keep Greavers cornered. In a big room this is not a good choice.

#4 Aside guns I also recommend a Leviathan Blade here that can either deal much damage or blow them away with charge (useful for first strikes) or Divine Avenger's 1st attack for its wide attack arc that.

-Gorgeus:

#1 That Snarbolax bomb (forgot name) will effectively interrupt these enemies with each blast. Just place one and move far enough away that they can't reach you with their 3 bites. Though I am not the bomber guy, I am agreeing that this weapon deals the best job vs Gorgeus and Wolvers (though it is boring with this bomb vs wolvers, but this is another topic)

#2 Blitz Needle's 1st shots. If safe, also 2nd or charge.

#3 Savage Tortofist is also a good choice since this gun can be used at extreme distance to the enemy and the randome floating around of Gorgous makes them very vulnerable for the placed shards since Gorgous can't dodge it like other monsters.

#4 Everything else that is spikey.

-Trojans:

Too lazy to think about it in deapth. Since the biggest problems come from other monsters of the named category, I have already choosen my loadout here. Well, but here is what probably the others would answer anyways:

#1 Blitz Needle: with max damage its charge can 1hit two trojans at the same moment in a 4 man group in d 27 and below (though I only got two at once only one time this far :P)

#2 Proto Gun: You just have to be patient.

-Healing Butterflies:

If you ever find yourselfe in a trouble with them, then I would suggest Dark Briar Barrage or by feeding them with poisioned cheese. If not, then just kill it with the weapon you already brought for the other monsters.

Ps: I write too much... it was meant as short answer (lol)

Personally I like to use my 4* giawatt (lol) because each of my expanded bullets do 198 damage and thats awesome, but thats just me.

You, sir, give kittens cancer.

The only times that are acceptable to use a Pulsar for DPS is against either Gun Puppy Equivalents, or Tortodrones. Otherwise, you will make everyone in the world hate you.