I hope you have got it figured out by now.
If not, here is an old post of what I had to do.
The trick was to cut or grind flats so you can get a real wrench on it.
Then a big cheater bar and Sloooooow steady pressure so you dont snap the dang bolt.
Ogre_fl wrote:
Well......That was a major pain. 
The wing leg screw design is a nightmare to get un-done after several years in saltwater use.
The threaded area is buried deep in the wing casting and of course being of dissimilar metals it was corroded together.
After breaking 3 Philips head impact driver bits and totally buggering the screw head I had to grind the head to fit a standard wrench.
Even with that it took a 3' long cheater bar. Thought I was going to snap that screw many times.
The wing leg screw design is a nightmare to get un-done after several years in saltwater use.
The threaded area is buried deep in the wing casting and of course being of dissimilar metals it was corroded together.
After breaking 3 Philips head impact driver bits and totally buggering the screw head I had to grind the head to fit a standard wrench.
Even with that it took a 3' long cheater bar. Thought I was going to snap that screw many times.
