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hm defog
The Hearthome gym leader Fantina lets you use surf in Pokemon Diamond, Pearl and Platinum.
If you are talking about Defog(HM05), then you will have to beat Crasher Wake first in order to use it outside of battle. It is somewhere in the Great Marsh. In Platinum, it's Fantina, and after you beat her, you can use Surf.
Use dark or fire type moves.
Use ground and grass type moves to finish him off.