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Went in figuring it would be fire since I pretty much always go with fire.
It's easy enough catching a grass type. You can usually find them hanging around route 01 or 02, and they tend to be pretty sturdy throughout.
Catching a water type is even easier, depending on how early you get a fishing rod, surf, or walk through a rainy forest.
Catching a fire type? Good luck. Your best bet is catching some bizarre normal type that learns Flamethrower.