BEARDED and with brightly colored tattoos on both his calves, Jon Shiner looks tough, but he admits he has struggled with heights his whole life. "If I have to stand on a chair I get nervous, so anything up in the air pretty much freaks me out," the 29-year-old said. But on a recent Saturday morning, Shiner decided to face his phobia when he attended a free class at Six Flags Magic Mountain on conquering the fear of riding roller coasters. The amusement park was hosting the course in conjunction with its promotion of Tatsu, a new dragon-themed coaster that officially begins running Saturday.