Don't stop jumping now!
Go to the level Barcelona (any game mode will work). Then go inside the building that you grind down while spiraling in story mode.
Get next to the wall that is opposite the staircase and get a running start and wallie off of the wall.
Since the wall turns you can keep wallie-ing if you get it right.
Eventually you will get to the top of the building!
We have more cheats and tips for this game here, Tony Hawk's Underground 2 cheats
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