So I want to make this tough guy who despises fiends of sorts, & I'm not sure whether to take it as a medieval doomguy or just try & go gunslinger. Or hell, (haha pun), Maybe not even fighter, idk, help? Thanks!

The first things that stick out to me about Doomguy are his guns and his finishers. For the guns, you'd want either the gunner or crossbow expert feats and the Archery fighting style. For the finishers, you'd want the Unarmed Fighting style and perhaps the Grappler feat.

An issue here is that you'd end up splitting your resources a lot if you want all these bonuses. You have two feats and two fighting styles you want. You probably want to go gunslinger, so that would mean you'd need a third feat to get the second fighting style. This build would also require strength and dex to both be quite high (Dex for your attacks and strength for your grapples). The hunger for feats here would make it difficult to boost your ability scores depending on what they start at. Of course, you could trim the build a bit, but then you may end up missing the mark for a Doomguy build.

Alright thank! that was enough help!

There's a couple ranger subclasses, monster slayer and gloom stalker, which can kind of fit a theme of hunting fiends. And paladins get bonus smite damage when they use the feature on a fiend, Oath of watchers could be a good fit. The problem would be pallys need to melee to smite, so using a gun or box doesn't work for them. Also, a warlock with a fiend patron could be a fun, kind of keep your enemies closer thing, and you could flavor your eldritch blast as a gun.

Quote from Xalthu >>

There's a couple ranger subclasses, monster slayer and gloom stalker, which can kind of fit a theme of hunting fiends. And paladins get bonus smite damage when they use the feature on a fiend, Oath of watchers could be a good fit. The problem would be pallys need to melee to smite, so using a gun or box doesn't work for them. Also, a warlock with a fiend patron could be a fun, kind of keep your enemies closer thing, and you could flavor your eldritch blast as a gun.

 I was focused on fighter because of where this was posted, but Xalthu makes some good points.

As an alternative, consider artificer. You'd need to put in some effort flavouring stuff, but either artillerist or armourer could work for Doomguy. I think artillerist would work more since it's easier to flavour their features like Doomguy, especially since one of the features is literally called Arcane Firearm. The Arcane Firearm could literally be a rifle, but with a rod instead of a barrel, that shoots firebolts instead of bullets. Artificers are also the only base class that gives the opportunity for firearm proficiency if you really want to use an actual gun.

Artificer would also prevent the need for str and dex to be high. You'd be using int for all your attacks, and you can grab the Armour of Magical Strength infusion which will let you use your int modifier for grapples. You may still want to grab gunner or crossbow expert so you can shoot with enemies in melee range, or grappler to attack grappled enemies with advantage. Technically if you just had the grappler feat then attacking a grappled target would cancel out the disadvantage and advantage.

Honestly, the more I think about this, the more I like the idea of Artificer rather than Fighter for this concept. I already have an armourer made, but now I need to make an artillerist sniper.

You can do a bit of all of these and multiclass