The shock mounting bolts might be quite tight, and there's often a chance they're seized in place. Give them a good old squirt with a penetrating lubricant before you start. While that soaks in, go and find the torque settings for the mount bolts, and dig out your torque wrench, breaker bar, and the correct sockets.
A gentle tap on a bolt with a hammer can sometimes be a good way to let corroded threads know you're coming, as it were. It gives them a jolt and increases the chances of the bolt turning when you begin your attack. Do the tapping via a socket and extension bar to avoid hitting anything you don't want to.