OP's problem here is that the half shaft is broken, so he has no way to keep the differential from turning while he tries to loosen the bolts. Car in Park or parking brake applied will not help him in this situation.

As Vstech stated, an impact gun is going to be your best solution since it will require much less effort to stop the halfshaft from turning. If you don't have an impact driver, you might be able to use a plumbers pipe wrench or a large pair of vise grips to hold the broken half shaft while you loosen the bolts.

It's critical that the bolt heads are completely clean and the driver bit is fully seated. Take a few minutes with brake cleaner and an ice pick to get them nice and clean.