The trick is to farm points. Buy the rebound ability as soon as you can -- that's 1000 points. Then buy the health upgrades just to be safe. Once you have those, getting points is easy. Here's what you wanna do:
Go to an arena, and find a spawn point for enemies. Don't close it. Instead, find a nearby wall, and wait for them to come out. Rebound of the wall, then slide, and go for headshots. Build up your multiplier to x5 this way. Once it's like that, you'll be getting more than 100 points for each kill, and they'll just keep swarming out for a while. Eventually you'll have to close the door to progress, but this will get you thousands of points for each arena. Some arena's aren't as good as others, but the one at the beginning of "Everybody Runs," for example, can easily get you 10,000 points alone.
Good luck with it, bro.
100% Retail Games: Tony Hawk's Project 8, Viva Pinata, Harry Potter and the Order of the Phoenix, X-Blades, Fallout 3, Tekken 6, Mini Ninjas
100% Arcade Games: Banjo-Kazooie, Banjo-Tooie, Arkadian Warriors, Shadow Assault Tenchu, Golden Axe