Tanks: Winston, Hammond, DVa, Roadhog. Winston is good because, in practice, Sym should be relying on her own shield. Hammond, DVa and Roadhog are on their dueling merits, roughly in that order. Hammond can boop her out from behind shields with skilled play.

DPS: dive heroes - Tracer, Doomfist, Genji, in about that order. Tracer dominates the matchup; she's a tiny target that even great Sym players will struggle to build charge against, and she can blink away from personal defense turrets. But Sym will likely play within her team, near her tanks, so she can be a tough Tracer target. Doomfist is better at disrupting her tank support and singling her out in that environment. Snipers are good too, if the map allows you angles on her. Sombra can remove her shields.

Supports: Lucio can boop her around, I guess? An aggressive Brig might be part of a team plan against a strong Sym, maybe? Zen and Ana can help in taking out any target, but her team's barriers will be a problem.

If you're facing some sort of dominating smurf Sym who is carrying your team, I'd suggest like Winston/DVa/Doomfist/Genji/Lucio/Ana or Winston/Hammond/Sombra/Widow/Lucio/Ana or something like that.