What kind of tool do I need to access this bolt?
I believe the tool to get a proper "bite" on the nut is a crow foot wrench attachment. You'll be able to get a cockeyed bite on the nut using a typical wrench in order to unscrew it from above, but just for posterity, a crowfoot wrench is used for these angles. I've also had deep sockets that are hexagonal at the end so that they may be used in a wrench. Seeing as how you cannot unscrew from the top, I suggest using a socket with a hexagonal end fitted into a flex head gear wrench.