I'm assuming it's a car with air conditioning.
Sorry this is al old thread but for anyone looking to to sort this one I can help.
You will need a small socket extension, ratchet handle and 13mm socket. I used 3/8 drive but 1/4 is easier to connect. It depends on having a long enough but not too long extension.
Access is from the top and is terrible (but you know that) so you will need a suitable wood batten to lever the socket handle and break the thread grip. Once its moving it will take ages and ages to get the screw out. You might also have trouble with the bottom bolt(s). The threads go right though so are at risk of corrosion seizure. Loosen the bolts then squirt some penetrating oil into the open threaded hole where its got more chance of doing some good.
The alternator come out from underneath. I disconnected the exhaust pipe. More work but well worth the effort to make some working space.
When you put it back together don't tighten the other bolts until you have that difficult top bolt threaded. Use a good anti seize paste all the way up the bolts. It will make removal easier next time and Sod's Law says you'll be doing it.