1. Level Up: Ensure that your Tepig is adequately leveled up, ideally at least level 17 or higher.
2. Effective Moves: Teach your Tepig ember and rollout. Ember is a fire-type move, super-effective against the grass type Pokemon used by the gym leaders. Rollout benefits from Tepig's high attack.
3. Status Moves: Consider teaching Tepig growl, a move that decreases opponent's attack. This can help you weaken the opposing Pokemon and make it easier for Tepig to defeat them.
4. Use Items: Carry potions or other healing items to restore your Tepig's health during the fight.
5. Strategy: Start by using growl to lower the opponent's attacks, then utilize Ember's STAB (same-type attack bonus) to deal super-effective damage. If your Tepig knows rollout, use it to hit multiple times successively, dealing significant damage.
6. Be patient: the battle with the first gym leader may take some time, especially with Tepig being weak to grass type moves.
Remember to be patient, adapt your strategy based on the situation and take advantage of Tepig's strengths while capitalizing on the opponent's weaknesses.