Freeing stiff nipples?
I agree with Daniel’s comment, I’ve had success with using vice grips to hold spikes myself. Be careful not to use excessive force to prevent crushing the spoke. It may help to add a small “flag” made of tape to each spoke to help visualize spoke vs nipple rotation.
Matt’s suggestion of using penetrating oil and heat is a good idea, provided you removed the tube/tire and rim strip first like Daniel suggested. Brass does in fact expand more than steel when heated.
Of course, be sure to use a high quality spoke wrench to prevent rounding the nipples.