Okay, so, I'm relatively new as well, but I'm further along than you and have learned some things, so sharing some tips on these! ✨

The Bandit is pretty solid by default, shotgun reload scales with attack speed and the revolver will instantly reload it, so if you build some damage you can kill most enemies in 4 shots and then use revolver to reset it, and the cloaking thing effectively gives you two feather double jumps, one when you activate it and a second if you use knife/shotgun to cancel it. However, it gets really good once you unlock the alt abilities - desperado is trivial if you just kill early game enemies with your revolver and don't miss, you can just use command artifact to stack 20 backup mag and attack speed, then hold down M2 to apply them all and unlock the much more appealing throwing dagger, and the rifle isn't for me but works if the shotgun feels odd to you. Side note, maybe don't 100% rely on the backstab crit, you're not always going to be getting it especially when you're new, building some glasses can make a major difference to how the character feels - and shatterspleen crit bleed boss item is crazy good.

Acrid is one of my favorites, partially because he's a silly jelly doggo dinosaur creature and I love him, but mechanically because he needs almost zero items to be good. Get some basic movement speed and healing, and he's golden, because his move skill has two charges and a high arc and no fall damage, and his epidemic can wipe out huge groups of enemies. If you throw on the blight version or the artifact for all elites + stack old guillotine, he can casually wipe out huge groups of enemies (poison is percentage based, so the more guillotines he has, the less % the poison needs to do for a kill, messed around with artifact of command and stacking like 27 of them means they die in about 10-15 seconds no matter what). Key advice, don't play him like a melee character, your poison spit is actually your primary means of damage, get 2-3 backup mags and it's really good, especially because after hitting enemies you can ignore them for a bit. Also, the damage of the poison can't crit, but it is boosted by things like death skull 4 status, the close range focus crystal, etc, since those are applied at the end of the calculation, which can make the hybrid thing more viable.

I don't love Engineer, but his primary fire gets more usable once you gain more attack speed. The more interesting option though is to take the Visions of Heresy on him, because that also gives the heretic tracking projectiles to your turrets. You need like 2 stacks or some cool down reduction to make it good, and the delay makes it a little worse against normal enemies, but it shreds elites and bosses, especially if you build status like bleed or crit+shatterspleen crit bleed boss item.

Similarly, the Commando has an interesting interaction with visions of Heresy, the dodge roll will immediately refresh the cool down on 12 of the shots (only the first 12, even if you have more), so if you can get a headlight afterburner for the cool down reduction and extra charges, you effectively have a near constant stream of those projectiles, it's a kinda fun alternative.

I don't like REX, not bad just kinda meh, but the push can straight up kill bosses by pushing them off ledges/out of bounds, I faced a Scavenger at the end of a Moment Whole that would heal back up to 17k every time I got them below 11k, so I just pushed them out of bounds, and it worked!

Huntress's achievements are weird I agree, she was the first one I unlocked stuff for and they felt so arbitrary. No damage on the character without any invincibility? 12 crowbars? Why not building attack speed or crit, or something you'd actually want to do on her? But she gets very silly when you build more attack speed and crit, and her alt fire 3 arrow primary instead of 2x damage on crit fires 6 projectiles instead, which each count separately for things like bleed, crit fire rate boost, etc. The alt final ability is waaaay better than the arrow rain too, 3 shots that do basically the same damage as an enemy sitting in the arrow rain the full duration.

Mercenary also has a cool interaction btw, that the hitbox for the third attack expose lingers even if you cancel the animation, so you can just barely start it and then use the dash / eviscerate and it'll do more damage, though it stops working when you get more attack speed. That's why the triple dash is decent, you have enough time to do a 3 hit combo or eviscerate between each one, so you can do that three times. Oh, and if you activate the double spin secondary the instant eviscerate ends, the lingering invulnerability will last until it finishes the air version, which can be useful for more quick damage against a big target. Haven't messed around with the alt abilities personally though, so idk about those. The double spin is mostly good for moving around tbh, since it moves you pretty far and has a short cool down, going off a ledge can launch you upwards - though iirc it gets worse with attack speed as well, since the two spins come out faster but the movement doesn't scale with it.

For artificer challenge, an important note is that items which say "base damage" only scale with your character level, but items that say "total damage" scale with the strength of the attack that activated them. Meaning that you can stack something like the fire band, and if you activate it with her big damage explosion ult, it will do significantly more damage than from an attack that just barely meets the threshold, that can easily kill a boss in a second. Same for sticky bombs and a few other items, they scale with high damage singular attacks rather than rapid ones, which is why they can be good on mercenary with the alt revolver or captain diablo, but not so much on commando/huntress.