IMHO, Rod has the best answer. If you don't have an air gun, try finding a friend to stand on top of the tie rod end to force the assembly down onto the arm to help hold the ball stud still while you turn the nut. Alternatively, use a big pry bar and figure out a way to pry that arm DOWN.
Once the ball joint is removed, carefully inspect the bore in the arm. It should be tapered and round (in other words, a perfect mate to the stud), not tapered and ovaled out. If the hole in the arm is worn, you'll likely not get the new ball joint tight. You'll either need to replace the arm or JB weld it.