First, soak the bolt with penetrating oil, in case rust or corrosion is a problem. If the head is in good shape, try using an impact driver or impact wrench. If the head is damaged, causing a wrench to slip, use an easyout. You can get an easyout kit at an auto parts store. For the latter, you will need an electric drill and bit of the proper size, and the easyout, which drives or screws into the hole made by the dril bit.

a 6 sided socket of the correct size, a good ratchet and perhaps a longer pipe over the handle of that rachette. penetrating oil may be counter productive since the bolt or nut will be slippery. heat can help.

Soak in penetrating oil (Not WD-40 but a REAL rust penetrant). Then vibrate the heck out of it with an impact wrench or several hundred hammer block. Vibration breaks up the rust and lets the penetrant work better. Read this article on the best way to do it

http://ricksfreeautorepairadvice.com/loo...

A socket wrench with the appropriate allen head driver and if necessary a piece of pipe over the handle of the socket wrench to increase leverage .

cheater bar or air gun.

thanks guys