They way I use the spanner wrench.

Hold the engine with one hand on the propeller shaft put the spanner parallel to the shaft and tighten the nut, try to use one hand to push the spanner and pull the shaft at the same time until you can't move it, this way I never got it to get loose. Also this method do not put any stress on the airframe, so even if you use a lot of force you won't break anything.

Regards

Alejandro