What are some good teams that can be built ingame (without trading) that can beat the leaders in single, double, and triple?
For a single battle:
Serperior, Archeops, and Haxorus
For a double battle:
Cryogal and Krookodile, or Darmanitan and Heatmor.
For a triple battle:
Krookodile, Electross, and Reuniclus or Hydreigon, Lucario and Heatmor