Use a 15mm pedal wrench. Only use the right tool. Don't try using a 9/16" wrench that just sort of fits. It can slip, and you can get hurt. (think about that big chainring). Most store bought wrenches won't fit between the pedal body and the crank arm. Use a pedal wrench. I've had success with a Snap-On 15mm wrench, but those are expensive and somewhat hard to buy.