Mabinogi works fine with Boot Camp because you end up rebooting your computer (that's the "boot" in "boot camp") in Windows. It's just like running a regular Windows computer, only on Apple hardware.
Parallels works a little differently. You're still running the Mac OS, but Parallels lets you run windows software. It's an emulator, just like the NES emulators people use to play old Nintendo games on their computers. The probelm is, when you run windows software in an emulator, graphics and sound have to go through Parallels to get to your hardware. That is, Mabinogi doesn't tell your graphics card what to display, it instead tells Parallels what to display, and Paralles talks to your graphics card. And since Mabinogi is looking for graphics drivers and sees Parallels instead, it can crash, refuse to run, or complain about graphics drivers.
Your best bet is to continue using Boot Camp.
