OK I did a replay again with Dusclops to see if my opinion would change.
Lv 46 Quiet Nature, caught at Lv 27 in Mt. Pyre
HP 102 (11 IV)
Atk 84 (23 IV)
Def 134 (18 IV)
SpA 79 (20 IV)
SpD 133 (13 IV)
Spe 40 (30 IV, lol)
This time I caught it before fighting Winona to give it more playtime. I reserved a Secret Power and Ice Beam TM just for it.
Mt. Pyre to Gym 6 (Lv 34, Ice Beam, Confuse Ray, Secret Power, Feint Attack): Off the bat, Duskull was a dumpster fire. Super underleveled and very frail to what the trainers use in Mt Pyre. In particular, there are quite a few trainer that use Night Shade, which is a hilarious counter to Duskull's godawful HP stat. After the grueling trek for the Shadow Ball TM, Duskull ended up at Lv 29.
Gym 6 : With the Ice Beam TM, Duskull could only manage 2HKOs with Ice Beam vs the trainers in the 6th gym and takes 4-6 hits before going down. Vs Winota at Lv 34, it barely misses a 2-shot Ice Beam vs Tropius and can 3 shot on Altaria. However, Tropius can 3-shot with Razor Leaf, and Altaria can Dragon Dance + two Earthquakes before Duskull can get a KO at full health, and even less if Duskull took any chip damage before. Very meh.
En Route to Gym 7 and Volcano Path(Lv 34, Psychic, Confuse Ray, Ice Beam, Secret Power) : I switched to Psychic because there are quite a few Fighting trainers. At this point Duskull can hold its own against unevolved mons, but it still takes hefty damage from evolved mons like Swellow and Manetric. Shadow Ball is also not very reliable since lots of Route trainers use Normal mons even at this point of the game, and Team Magma/Aqua have lots of Dark mons. Zubats are also extremely obnoxious with Confuse Ray + Super Effective Bite flinches, as are Mightyena with Swagger.
Gym 7 (Lv 38): Finally, Dusclops can One-shot some trainers. The Kadabras, Ralts, and Natu lines all go down without a hitch. The Girafarig line is annoying, though. Vs Tate & Liza, Shadow Ball can 2-hit the Xatu, although the Xatu+Claydol are more likely to get the KO first on Turn 2, so it's pivotal to have another mon that can double team the Xatu on the first turn (I had Slaking). Dusclops can also 2-shot Lunatone and 3-shot Solrock. Dusclops can only tank four Earthquakes from Claydol, however, and all mons move before Dusclops, so it's very likely that it gets KO'd before it can get in two attacks. A Xatu start with Confuse Ray or Hypnosis could also be a death sentence. Decent, but mixed results.
En Route to Gym 8, Maxie Final, and Archie Final (Lv 39): Dusclops is not great vs Maxie. The Golbat and Crobat eat it alive with Confuse Ray + Bite, and while the Mightyenas can't deal any damage, they are very annoying with Swagger. The Camerupts can be dangerous if they ever decide to use Rock Slide or Earthquake, although Maxie enjoys using Amnesia for whatever bizarre reason, so there's that. Vs Archie Crobat is a threat, although the rest of his team can only Confuse harass Dusclops. Outside of rain, the Route trainers aren't much but with rain they can threaten to 3-5 hit Dusclops, so it takes quite a few potions to get through after the Archie match.
Gym 8 (Lv 41, Psychic, Shadow Ball, Confuse Ray, Will-o-Wisp): Dusclops is not a fan of Lovedisc's speedy confusion, and thank goodness my Dusclops was female because I'm sure the Attract would have made this match unwinnable. Otherwise, Dusclops can 2-hit with Shadow Ball. It can take down Sealeo fairly easily as well with 3 hits, although if it took any chip damage Sealeo could get the KO first. Kingdra is a big no since it can Double Team + Rest right in Dusclop's face without much worry, and Whiscash in Rain can KO Dusclops faster than Dusclops can do the same. Overall poor
En Route to the Elite Four (Lv 43): Gardevoir goes down with 2 Shadow Balls. However, Gardevoir can 3-hit with Psychic, and if it gets the Double Team ball rolling, it can take down Dusclops first. Roselia also goes down in two hits, but Toxic+Leech Seed can hose it. Delcatty is annoying, and Magneton is a losing matchup. I didn't have Ice Beam for Altaria, but I imagine Dusclops could have 2-hit Altaria as Altaria could only deal a quarter damage with Dragon Breath. Overall, Victory Road is a mixed bag, though generally favorable for Dusclops. I didn't have Psychic in my last playthrough, and I think Psychic+Ghost coverage is better for this place than Ghost+Ice. I can't say that Will-o-Wisp has done much of anything, however. I used it to decent effect vs a Slaking in the end, but that's it. Otherwise, it wasn't much help.
Vs. Sydney (Lv 46): No.
Vs. Phoebe: Phoebe has higher levels and potion spamming in her favor. The best Dusclops managed to do was KO the first one after a Curse, but that's really it. Everything else tore Dusclops apart.
Vs. Glacia: The first Sealeo goes down without effort. However, the two Glalies hurt hard with Crunch and Shadow Ball, as does the Walrein with Ice Beam and Surf.
Vs. Drake (Lv 48, Ice Beam, Shadow Ball, Confuse Ray, Will-o-Wisp): Bulk is irrelevant here. Drake's mons easily 2-3 shot Dusclops, and Dusclops can only get 3-shots even to mons double weak to Ice.
Vs. Champion: At this point, Dusclops is relegated to Switch-in fodder. Dusclops just can barely tickle anything, and it goes down in 2-3 hits against everything else. A high Def and SpD stat mean very little when you're fighting mons ten levels over what you have.
Overall: OK, maybe Duskull isn't the worst mon I've used, but it's still freakin awful. Its bulk is supposed to be its strong suit, and vs Route Trainers you do notice it. However, vs Major battles, all that amounts to is getting KO'd in three turns rather than in two. And what can Duskull do in that extra turn where it's alive? Not very much at all. Confuse Ray is good, but inconsistent. Sometimes it saves your whole match, and sometimes it ends in 2 turns without having done anything. Then there's Will-o-Wisp. Not only does it come very late, it also doesn't have very much impact at all. Special attacks are more common than Physical ones later on, and most mons have a good split of physical and special moves where they're not entirely out of a match for getting their Attack stat hosed. On top of all this, Duskull is a TM Hog, is really weak at base form, and is super vulnerable to status effects. The two highlights are its passable (though not great) match-up vs. Tate & Liza and being a competent Psychic+Shadow Ball user, but that's the extent of it. Is Duskull the worst mon available in Gen 3? No. Do I still think Duskull deserves to be F-Tier? Yeah.
