This team is OKAY for challenging Brock. No ROck-type moves at his disposal, so just carry a potion or two with you, and you'll be okay.
WIth this team, though, take advantage of Nidoran's Leer and Charmander's Growl. Use Charmander's Ember to take out Geodude, and against Onix, use Charmander's Growl against Onix two or three times, or until death. This will ensure that Onix's attacks do next to nothing. Next, just switch in Nidoran, use Leer and Horn Attack as you see fit, and stomp him flat. Make sure you use Leer on Bide turns.
It's a dragged out strategy that may be able to be avoided just by attacking full-on, but Onix's Tackle can do a bit of damage after a Screech. If you want the easiest alternatives, catch either a Mankey or Caterpie. Mankey learns Low Kick at Level 9, and Butterfree will learn Confusion at Level 12.