I have the tools and books to check the instruction of how to remove the cassette. I also checked BicyleTutor.com before I tried. But with all my effort, I just couldn't unlock it.

I need to use my left hand to turn the wrench counterclockwise, but I just can't apply enough force (torque) with my left hand. I asked my male cycling friend, who said it shouldn't be difficult to remove the cassette. Is it just me?