Enjoying this so far, though I'm kind of hoping that not every pokemon you catch ends up with a fascination for tearing the organs out of living victims.
As for level, depends on which mons you use. Grass is x4 effective to both of Brocks pokemon, and fighting moves are x2. If you use Biollante and Pcebro on him level 15 should probably be more than enough. If you plan to use the others...probably higher.