Wow, I wouldn't have bothered if I knew it wasn't going to kill him.
Go, Treewig! Start by hitting him hard with a Razor Leaf, then use your Superpower, and then use Mega Drain. If he uses Flamethrower, then curl up into your shell and Protect it, but never twice in a row.
Razor Leaf-> Superpower -> Mega Drain
IF Shadow uses Flamethrower (WTH that thing can learn Flamethrower and Hydro Pump), THEN use Protect, but never use Protect more then once in a row.