When I replaced my mine a couple of years ago, I was in the process of removing the entire sub frame and stripping the car back to the firewall. I was very fortunate that mine all came out withouth any real difficulty at all. Just had to lean on them with a breaker bar. (As I recall the torque spec on these bolts is about 80 ft-lb, so you hae to exert a little effort to overcome the torque, rust, etc.)
In my case, the engine was already out of the car. Consequently, I was able to get to the front body mounts at the firewall and liberally spray penetrating fluid on the bolt/nut and let it set for a day before trying to break it loose. The rear two are a different story, you can't directly access the nut, so I wasn't able to get any penetrating fluid on them, but they did come on out without spinning the captured nuts.
I was fortunate. Once those bolts were out so I could inspect them, I could see how bad they were. They were pitted, corroded, an in pretty sad shape. Definitely time to change them. I was lucky - bolts didn't break and nuts didn't spin.
Good luck with yours.