Shifty is a dangerous sweeper in the Sun. Thus, I would say that, because of that, it performs better than your typical middle evolution. Performance depends on more than just base stats. You have to consider abilities and moves too. Shiftry has good abilities and a vast movepool. Also consider that starter middle Pokémon tend to have better stats than non-starter middle evolution Pokémon, so it might not be the most fair comparison. Shiftry might not be the best competitive Pokémon, but it is great for a playthrough if you didn't choose the Grass-type starter.
Agrees on abilities and moves mattering. Hinges on making them matter. How often does Sun make sense for a team in a playthrough setting? Maybe in certain monotypes.
Certainly could make it work. Picks up all the necessary moves to sweep (Sunny Day + Swords Dance/Nasty Plot), given the right setup opponent. Requires foreknowledge. Would still be leery of the all-in-one setup. Faces probably two unresisted (Normal) attacks with iffy bulk. Prefers softening up with some Charms/Eerie Impulses + status.
Also agrees on Shiftry being a decent substitute for the Grass starter. Probably wants a Grass-type to handle Water, Ground, and Rock. Compromises none of those type match-ups with its Dark typing. (Honestly disliked Empoleon's typing for doing that. Netted it tons of resistances at the cost of being worse against Fire, Ground, and usually Rock. Became a good Steel Pokemon, but a rough Water Pokemon, if that makes sense.)
Sidenote: Could dominate the early-game in Shield with this thing. Acquires a Leaf Stone in Turffield. Fully evolves before the first gym. Visit the Pokemon Center for free Leaf Blade, Sucker Punch, and Sunny Day. No grinding necessary, unlike with TRs.
Not necessarily. In Singles, you can open with a Pokémon like Groudon or Torkoal holding a Heat Rock and switch in to Shiftry when it's safe to do so. Regular Ninetales can have Drought as well, but I personally wouldn't recommend it because Ninetales is better for setting her own Sun if that makes sense due to it being more of a sweeper than anything. Torkoal is perhaps the best since it's possible Groudon be banned depending on the rules and Torkoal can also learn the move "Yawn," which puts the target Pokémon to sleep at the end of their next turn. This either forces the trainer to let their Pokémon sleep or switch out. This allows you to safely switch in to Shiftry. Not much is outspeeding a Chlorophyll Shiftry in the Sun unboosted in some way except for Regieleki. In Doubles, you can just open with a Groudon or Torkoal with Shiftry as its partner right next to it.
Specified "outside of constructed teams and Dynamax" for that reason. Would not count on getting Groudon or Hidden Ability Ninetales in a normal playthrough. (Admittedly switched between thinking in terms of regular playthroughs and constructed teams a few times.) Leaves just Torkoal. Hopes you have access to it.
Faces a lot of competition for Sun sweeper. Sits about on par with Victreebel. (Slightly higher offenses, lower speed, smaller movepool, but also Weather Ball.) Says nothing about the higher regarded Chlorophyll sweepers (Venusaur, as you mentioned).
Cacturne is actually very good in Trick Room, but as a sweeper/revenge killer with a Focus Sash. In fact, it's the only way to use Cacturne, really. I've made Pokémon like Groudon and Kyogre work in Trick Room and they both have a base Speed stat of 90. Then again, that's more because of who their typical opponents are. In the meta game, Pokémon are typically much faster than the average Speed stat of 78. Even though 90 is not bad at all for Speed, many of the top-tier meta Pokémon are typically faster, allowing Pokémon with lower Speed such as Groudon and Kyogre to thrive in Trick Room against them. Thus, you can't really say for certain what Pokémon are too fast for Trick Room unless you know your typical competition. However, Cacturne would always be great in Trick Room because 55 base Speed is always slow. (Keep in mind, the average base Speed stat for all fully-evolved Pokémon is 78). I would argue that, on average when not considering the meta game or anything while playing under limited rules, you should really start worrying about being outsped in Trick Room if your base Speed is around 70+.
Stands corrected on acceptable Trick Room speed. Misremembered them as being slower. Happens to be even with Machamp. Was definitely slow enough, particularly with an appropriate nature and IVs.
Concurs with Cacturne having few options. Is that enough offense for a Trick Room team, though? Considers 115 offenses without an offensive item OR offensive ability underwhelming next to other Pokemon.
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Wants to bring this back to Dark/Grass a bit more. Which gyms and major fights would Dark/Grass be good or at least decent?
- Grass: Fine. Resists Grass. Only deals with Normal attacks otherwise. Trades neutral attacks.
- Water: Dominant. Handles Water and Dark well. Prepares for mainly Normal attacks again.
- Fire: Big no, for obvious reasons.
- Ghost: Mostly good. Dislikes Gengar at the end. Could set up easy-peasy against Yamask, however.
- Fairy: Terrible. Faces a super-effective move on every Pokemon.
- Ice: Poor. Hits the Grass side hard. Might be able to pop in against Lapras with a good switch. Dukes it out with super-effective STABs on both sides.
- Dark: Alright. Receives not very effective damage against everyone except the lead (Scrafty).
- "Dragon": Not great. Hits Gigalith and Sandaconda hard at the cost of being hit hard back.
- Oleana: Mostly bad. Rips the Grass side apart. Deals with Milotic easily, if needed. May be able to smash Froslass too.
- Rose: Mixed. Stay away from Escavalier and Ferrothorn. Swaps into a Shadow Claw, Throat Chop, Wild Charge, High Horsepower, and Zen Headbutt well. Battles it out with neutral attacks on both sides.
- Leon: Mixed. Not unsurprising, given the coverage of Leon's team. Switches in well in some situations. Requires more finesse here.
- Dragapult: Switches into Shadow Ball/Thunderbolt fine. Follow up with a STAB Sucker Punch.
- Mr. Rime/Seismitoad/Rhyperior: Comes into a resisted (or immune) attack, potentially, on all of them. Nails two of them on a double weakness.
- Inteleon/Rillaboom: Resists everything they have.
Handles itself decently. Naturally becomes less straightforward "good" or "bad" as coverage grows. Comes down to combat ability at times. Can you defeat [X] with equally effective attacks before fainting? Also helps to have foreknowledge. Becomes scarier if you cannot guess what move your opponent will use.
(Typed a lot more text than planned.)