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!!!MAY CONTAIN SPOILERS!!!
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From GameFaqs:
If you have Mario Golf: Toadstool Tour, you can transfer Neil and Ella
to the GameCube to earn experience points and compete in Toadstool Tour.
Here is the necessary equipment to link the two games:
1 Nintendo GameCube
1 Game Boy Advance
1 Copy of Mario Golf: Toadstool Tour
1 Copy of Mario Golf: Advance Tour
1 Nintendo GameCube/Game Boy Advance Link Cable
Plug the NGC/GBA link into the GameBoy Advance and connect it to the
GameCube. Turn on the GameCube with Toadstool Tour and wait at the start
menu. Turn on the GameBoy Advance and you will see a red connection screen.
Now you can transfer Neil and Ella to Toadstool Tour.
Neil and Ella will always be availible to use for Toadstool Tour.
The only time you have to link the systems is to transfer experience points
into Advance Tour. Just go to the Link Icon at the main menu and connect.
You gain experience for almost anything you do in Toadstool Tour. Some quick
and easy ways to gain points are:
Winning Tournaments (the harder the course, the more points you get)
Playing Near Pin (180 points for just playing 9 holes)
Character Match
These are the ones that rack up the most exp, points. Like I said, you get
points for doing almost anything, but might be only 7 or 30.
When you transfer data for the first time, you unlock Wario, Waluigi and
some courses for Advance Tour. Compete some more, transfer again and you
will unlock Luigi.