Everything below is just PvE, not PvP

Based on your playstyle, Nameless or Justifier would be right for you. It is a damage boost to have med ASI instead of low dmg (based on how close ASI and damage bonus are) but ASI helps a ton in the dodging part. Damage bonus is still going to be considered the best for damage output due to the nature of this game (3 shots to kill instead of 4, etc. etc.). Regardless, a low damage bonus is not very beneficial in comparison to medium ASI.

Personally, I use a Justifier / Nameless combo and never switch armor unless I am doing a shadow lair or a devilite / dark city / greaver level (which in that case, I usually go sword hybrid for shadow lairs). I can solo FSC just fine with the dodgey ASI + my ASI UV's that put plague needle at Max ASI and then my sent / AP at ultra ASI with a low asi each. Not having fire resistance / having piercing resistance armor doesn't really matter once you can dodge.

For Dark Cities / greaver levels, I would make sure you have a toothpick around, as well as whichever gun you are most comfortable with (Personally, Hail Driver / Polaris, Callahan if they are not packed). For armor I just throw on Divine Veil and Deadshot Mantle to hit maximum damage on fiends and undead (devilites, greavers, and kats. It might as well just be a med damage bonus per piece how useful it is in dark cities).

Having ASI is vital for a Blitz Needle or Plague Needle (more so than CTR).

Having no ASI feels weird for any gun, honestly. I would consider getting something like a Shadowsun Slicker for looks though :P

The issue with using Polaris as your main elemental gun in PvE is that the gun is designed for either knockback -> survivability, to kill gun puppies or other stationary things, and finally Devilites. If you spam Polaris on, say, a scuttlebot or any other every day clockworks monster, that is just outright silly. It mostly annoys your group because of the knockback. There are some instances where you will still use the Polaris regardless for its knockback in order to keep things away from you.

With that, It is in your best interest to keep with you both Polaris and another elemental gun for a 4 weapon slot set in my opinion (mainly for use on gun puppies).

For a 3 weapon slot set, I would not use Polaris at all.

Argent Peacemaker vs. Elemental alchemer in PvE is kind of tough atm. Argent is absolutely better in single target damage, while alchemers are AoE oriented and hard to predict with the splits. If you do it right, an alchemer can sometimes hit up to 6 times (that happens when aiming between two close monsters. The initial, split, and ricochet all three double clip = x6 normal damage). Personally, I am liking the new argent, especially hitting max damage against undead with the two med dmg trinks. Zombies just die really fast.

There are other places where alchemers shine, but all specialty uses.

1. Side clip the charge attack on Vanaduke repeatedly with a Hail Driver on first or second body phases. He remains immobile while you nuke, making probably the absolute easiest nuke, but not quickest. It is still slow in comparison to a needle, which doesn't require much skill on first / second body phases anyway, so this is not really as shiny as it might sound at first. But fun.

2. Dark cities. When kats dash at you, shoot an alchemer bullet straight at them to have at least the split, possibly even the ricochet clip (2 hits / 3 hits). Devilites are also quick to kill and easy to lock down with a Hail Driver. I would not suggest doing the kat dash trick with zombie leaps... anymore... -.-

@Swiftstrike Buckler, I would not suggest using it if you can achieve ASI UVs on your guns. With a med + med + low or med ASI UV totalled from gear, the high ASI isn't very worth it. You might as well improve defenses rather than have an either useless or low ASI bonus with bad defense.

Even if you aren't shooting for ASI UVs, med + med from armor makes low ASI cut off from the swiftstrike, making it less useful.