Stall teams in 6v6 singles generally work by switching in a Pokemon that walls your Pokemon, forcing you to switch, and taking that opportunity to use moves that give themselves a further advantage (usually hazards, hazard control, status, or knock off). If you want to defeat them, you need to prevent this from happening, and the easiest way to do that is with pivoting moves. Pivoting moves let you switch immediately after they switch, instead of waiting for the next turn, so your Pokemon still checks theirs and they're never able to safely do anything.
National Pokedex teams often run 3 offensive Pokemon that you can use as wallbreakers, and 3 defensive pivots to bring them in safely. Good defensive pivots include Galarian Slowking, Zapdos, Alomomola, and Pecharunt. You can also put pivot moves on more offensive Pokemon, like therian Landorus or wellspring Ogerpon, to force 50/50s.
Another thing you can do is look for vulnerabilities in the opponent's team. If they rely too much on one Pokemon to wall one of your Pokemon, you can try to set up some hazards and then lure in that one Pokemon as often as possible. Stall teams usually have a difficult time recovering if they lose just one of their 6 Pokemon.
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