Falkner's easy to beat. Just catch a Geodude on Route 33(or whichever it is near New Bark Town) and LV it up a bit. Battle trainers and wild Pokemon anywhere. He's really easy to defeat.
Route 32 is the best, if I was about to train for Falkner.
Falkner is incredibly easy to beat. Just train your Pokemon to even Lv. 10 (or Lv. 15 to save trouble for Bugsy, she is hard) and you'll smash Falkner.
Hope I helped. :)
The Onix for a Bellsprout is a good trade as well, it gets Rock throw at level 14. Even if you can't get a Steelix, it is still a worthy Pokemon.