Eventually, you'll reach a clearing where a Go-Rock Squad leader is attempting to capture a powerful Pokémon, likely a Snorlax. This will be your first 'boss' capture. Snorlax is a large, slow-moving Pokémon, but it has a lot of HP and can be difficult to circle without it breaking your line. It will also try to stomp, creating shockwaves that can damage your Styler.
Strategy: Focus on drawing large, continuous circles around Snorlax. Use your Partner Pokémon's 'Electric' Field Move to stun it, giving you a window to draw more circles. If you have any Pokémon with 'Tackle' or 'Pound' Field Moves, they can also help by briefly stunning Snorlax. Keep your stylus moving and don't lift it until you've completed a full circle. After successfully capturing Snorlax and foiling their plans, the Go-Rock Squad will retreat. Report back to the Ranger Union to complete Mission 3.