Bugsy is the second gym leader, and specializes in Bug-type Pokemon. So, in order to win, you need high levels and super-effectives.
So, you seem to have trouble with scyther - so I'm guessing Metapod and Kakuna aren't much of a challenge.
Scyther is lvl. 17, so, I'd say you should have your Pokemon lvls 20+. Scyther is also weak against, fire, electric, ice, flying, and rock. Luckily, your sandshrew can learn rollout @ lvl7. Also, you should try to catch some other Pokemon who can definitely help - as far as I know, you only have 2 Pokemon since you're not using your starter.
Hope I helped! :)