I came into top 363 ladder, and I didn't even want to.
I just played Sinistcha, and this mon destroyed almost every team by its own.
I don't see any counterplay to it with tera poison, and this mon is even better in a veil metagame.
Nothing ohkoes it, or it can just tera.
It can also be healed so easily and get some burn!
Here is my last game:
https://replay.pokemonshowdown.com/gen9uu-2010656604
I think that the only way to win against sinistcha is to use taunt or encore.
(Sd scizor is good but can be burned)
Sinistcha is definitely really good, and it's probably better than ever right now between the strength of screens, Dirge rising, and Hands getting banned again. Colbur sets have always felt particularly brutal to me to bait Knock Offs and turn would-be checks into setup fodder without committing to Tera. But it's still got its fair share of counter-play.
Encore, as you mentioned, is your best option, and it's extremely powerful in general this gen with all of the setup `mons that terastalization empowers.
Sinistcha doesn't like strong special attackers (Hoopa-U, NP Torn, Specs Drei, Tera Ice Sandy Shocks, Goltres, Tera Flying Thundurus-I, etc) as long as they switch in early, as even strong neutral hits can keep it from CMing out of control and put it in position to be revenged. And there are a few CM sweepers who can beat it at its own game (Cress being the most viable, but there's some fun fringe stuff out there, too).
Physical attackers are mostly relegating to revenge killing Sinistcha, but there are a couple options that can get the job done from full or near to it. LO/EBelt Mamo doesn't need much chip to break it if it's pre-tera or if it's tera poison/steel (you just need to bait the tera first). Iron Leaves needs to Tera out of its Ghost weakness to win, but SD boosts will accumulate faster than Strength Sap drops and it has the bulk to stand up to a boosted hit or two, you just need to know your magic number and not get burned (one of the reasons I like Tera Fire). Shockingly, most of our other SD Sweepers don't have much to offer against 'tcha between frailty and needing too many boosts to get through.
If you find your team really struggles with Sinistcha, though, my favorite 'mon by far into it is physical Sap Sipper Hoodra. It blocks both Strength Sap and Matcha Gotcha healing, can eat even boosted Shadow Balls pretty easily, can Knock lefties, phaze with Dragon Tail, and/or wear it down with Heavy Slam (especially if it grabbed an attack boost by switching into a predicted grass move). And if you want a
truly cursed option...
Most/all of this counter-play is obviously altered by screens and/or tera, but that's no different from any other setup sweeper in this frequently infuriating generation
¯\_(ツ)_/¯.