Make your way back to New Bark Town and talk to Prof. Elm inside of his lab. He'll give you... a Master Ball! Awesome! The Master Ball will catch anything without fail, but you will only have that one (most likely), so you'll need to use it wisely!

After talking to him, you'll need to go back to Ecruteak City before you can head east of New Bark Town onto Route 27. You have to do this, and it's going to be a bit of a tough fight, so make sure your Pokemon are healed, you have plenty of Hyper Potions or Moomoo Milks, Revives, and fighting spirit!