I cannot get to the last upper Bolt on the starter?

Do you have any tips/suggestions for accessing the last bolt on the top of the starter? I removed and got to the supposed hardest one, everything underneath the truck but there's one you can't see. I watched several videos. Some said use a mirror, others said a long extension until you feel it snug. Diagrams and detailed pictures would greatly help as I'll be back at it tomorrow. Thanks