As someone else said use a good socket with an extension and breaker bar. Preferably 1/2 drive. That should give you a 90 degree angle and plenty of down force. Using a torch first works on most rusty bolts. The expansion and contraction usually breaks the rust. We used to call it "heat and beat". If you haven't rounded the corners of the plug, try a few hard raps with a hammer on the breaker bar to jolt it free after heating. If you have rounded the corners and can't get a grip drill the center and use an easy out.
Oh ya, I forgot. Try an impact wrench if you own one. They will often jolt a rusty bolt loose.