get the oil pan gasket off of the car

I am working on a friends car and have done just about everything I could to get the oil pan gasket off of the car. I took out the 13 10mm bolts but there is a 13mm bolt on the rear right corner of the oil pan which is impossible to get to because of the transmission. All website I have been on talk about the 13 bolts I have already taken off but no mention of the 13mm bolt that is out of reach.