Where are you in your Black or White playthrough?

[Link]

I have sandile. I will train it higher so that I may succeed in the gym.

[Poryhedron]

I have sandile. I will train it higher so that I may succeed in the gym.

Advice: bring both Sandile and a Blitzle (or preferably a Zebstrika). Ground-types work great against the Gym Leader's own Zebstrika, but there are also a few Emolga in the Gym, and they're immune to Ground. Since Emolga takes standard damage from Electric, a Zebstrika of your own can mow them down; the Shock Wave attack makes a great answer to an Emolga who's abusing Double Team.