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She has a level 18 Clefairy and a level 20 Miltank.   Keep on following the trail until you see an odd tree in the way. You can also reach Route 36 by going east nearby the gate leading to National Park on Route 35 and using Cut on the tree by the Bug Catcher trainer.  You don't have to capture the Sudowoodo. Just make sure that you somehow manage to make it go away. There is only one Sudowoodo in the game unless you trade with someone, so that's basically the only time you get to see one. Be sure to save before battling it, so you can restart if you fail by being defeated or by defeating it (if you wanted to catch it). Sudowoodo is a Rock type, not a Grass type (even though it looks like a tree). He will then give you the Rock Smash TM.

Summary:
Get to Goldenrod City. Defeat the gym leader, Whitney. Go inside the house that is either next to or above the gym, and there should be two girls inside. Talk to the girl at the table, and she should give you the Squirt Bottle. Go north and when you reach National Park, go east to Route 36. Use the Squirt Bottle on it and a level 20 Sudowoodo will attack you. Battle the Sudowoodo. After defeating, capturing, or fleeing from the Sudowoodo, follow the trail on the right and talk to the chubby guy that is closest to you.