Water it with the wailmer pail, and it attacks you. (It's actually a level forty something sudowoodo)
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you have to go to certain towns like the tree play
its not an emerald its RAYQUAZA the main legendary Pokemon in Pokemon emerald.
If you go through the dessert in Pokemon emerald, Sapphire, or ruby, you'll see a boy standing next to a tree. Talk to him and he'll give you the TM Secret Power.
if you mean sudowoodo go battle frontier and use wailmer pail to the weird tree in south-eats near the waterfall
Use your watering can on it. The shaking tree won't like it and it will be a Sudowoodo try and catch it then you will be able to get into Smeargle Cave.