Sometimes knocking it in place with a hammer is enough to allow you to torque up the nut.

 

You have to hold tension on the tapered shank of the ball joint to stop it from turning, one way or another.

 

And make sure there is NO grease/dirt on the tapered parts, on the ball joint itself and in the hole of the arm.
Clean them with some brake cleaner on a rag.
The tapered parts should "grip" with some pressure.

 

prybar should get job done.

 

wow just a little brake cleaner on the bolts and they froze right up to tighten perfectly go figure.