Heres my thoughts:
I agree with whats been said about the rifle. It seems redundant atm.
The nailgun is pretty cool, if only the weapon sprites were bigger, it looks as if it is a pistol atm. And what would be really cool, is instead of having the nails become bulletpuffs when the they hit the walls. Have them stick to the wall for a while instead. Imagine the epicness watching the wall full of nails behind the monster pack you just ripped through!
The flamethrower, It sort of works. However the flamesprites look horrible in my eyes, definitely do something about that. Also, its going to have to be nerfed quite a lot. Spraying any monster except the Cyberdemon, Spidermastermind and Bruiserdemon absolutely stunlocks every monster by 100% giving them no chance to do anything. Making the weapon as powerful as a bfgclass weapon. The damage it does could maybe be reduced a bit too. It brings down stuff pretty quickly.
I'd suggest a bigger but less opaque scorch for the projectiles too.
The Darkclaw. Meh, the sprites are really a no-no. And the primary fire is really lame. The altfire is somewhat ok though, If this weapon in some form stays, I'd suggest ditching the primary fire and using the altfire for it instead. Making it some sort of leech staff entierly might work. Maybe using some old(or new) DE sprites.
A new slot6 weapon could IMO be a sort of railgun that shoots a fast bouncing ripper laser, you could use it to pick off monsters behind corners. I think zen dynamics had a similarish gun. I think i've made one a while ago, I could fix it up and post it here. It could maybe use this sprite if NMN is ok with that.
Regarding the monsters, I think they are pretty good. I like the fleshwizard, I don't think its more annoying than the painelemental because the fleshspawns are significantly weaker than lostsouls. Maybe drop the wizards health slightly too to make it less annoying.