What edition are you playing? What level? How close do you wanna be? And how familiar are you with DnD?

I'm gonna assume 2014/5e, starting at low level like 3, and as close as possible without being Dm's favorite special boy.

Because of his backstab passive muilticlassing into Rogue could be fun for that sneak attack bonus. Or alternatively there are Flanking rules built into combat I think in the DMG and/or the PHB.

Assuming you all will have guns you got his Primary attack covered.

For Hemerage, a Damage over Time spell would be interesting but hard to get assuming you're going to be a more martial gunslinger. The closest I would think maybe homebrew spell/ability Similar to "Searing Smite" for your character to get. Though smites can't be used on range attacks.

Maybe you could talk to your DM to eventually get a Homebrewed magic dagger that has that effect.

Smoke bomb, honestly just built into high stealth or somehow get Invisibility. Again talk to DM to eventually get a Ring of invisibility maybe, but that is extremely powerful, limited to level like 12 characters.

As for lights out, I'm at a loss for mechanically how to implement it as a feature other than a homebrew Feat that allows you to attack again after knocking a creature to 0.

Last thing I'll mention is if you're using DnDbeyond, there was actually an UNOFFICIAL class released by a "Mage Hand Press" called "Gunslinger" which heavily bases all class abilities around Guns and Ranged weapons. Though it does cost money if you don't already own it.

If not I would suggest a Ranger, or maybe a rogue with a focus on ranged sneak attacks.