Sneasler
Sneasler is already fast at 120 base Speed, and Unburden doubles it once its held berry is consumed — after that it outruns essentially the entire format. Dire Claw (its signature) can poison, paralyze, or sleep, and Close Combat plus its Poison STAB give it the coverage to break past most walls once it's moving first.
Don't let it get the free Unburden turn. Once the berry pops it's twice as fast and you've lost the speed war — so pressure it before it activates, or bring a bulky answer that doesn't care about being outsped. Also don't spam contact into a Dire Claw set and hope: the poison/para/sleep chance can end your attacker's game on its own.
Corviknight resists Poison and takes Close Combat without folding, and it doesn't hand Sneasler an easy KO to snowball from. Speed doesn't matter if the hit doesn't dent you — and Corviknight pivots away to reset.
Hatterene resists Fighting and threatens a Fairy/Psychic hit back. It's slow, so it's happiest under Trick Room, but even outside it a bulky Hatterene forces Sneasler off Close Combat and blunts the sweep.
You beat an Unburden sweeper by not letting it outspeed the turn that matters. A faster answer, Trick Room, or a priority move flips the read — and Gallade's Psychic STAB is super-effective into Sneasler's Fighting/Poison.
Sneasler wants a partner that forces the berry-activation turn (chip it to a Sitrus proc, or Fake Out cover so it sets up safely). Deny that free turn — remove the Fake Out partner or keep Sneasler from ever dropping below its berry threshold on your terms.
Every answer on this card was checked against the live learnset and roster data shown above — a citation that no longer holds is dropped, not shown. Type-based answers are computed from base stats and STAB. Speed is the real level-50 maximum. This is deterministic and sourced — no guesses.