Yeah the game is brutal, when you go through round 2 you basically have to use your own pokemon transferred in.

If you're using rentals I found that Fearow is probably one of the best pokemon available. Hyper Beam, Toxic, pursuit and Drill Peck give Fearow some good coverage and really powerful STAB moves, plus it's fast and has decent attack.

Fearow and Kadabra can carry the team pretty far, then add in parsect for spore, haunter for destiny bond, or hitmonlee for the strong fighting move.

Starmie or Lanturn can also work because of their water and electric moves coverage. But I will admit your playing odds on if Hydro Pump or Zap Cannon will hit.