Reach in the wheel well with one hand and hold the shock by the dust boot. Reach in the trunk with the other hand holding the impact and buzz the nut down. Don't overtighten it.
I can't remember how much thread is there but if you can fit two nuts on top of the main one you could use the middle one to hold the rod from spinning.
Most of the time there is either a flat spot on the shaft, or an allen key where the nut goes on to keep the shaft from spinning. Or if you want to get super ghetto vice grips and a shop towel on the shaft...