Just for the record; "Stripped" means the threads are buggered. If the points on the hex nut are compromised, then the bolt has been "Rounded".
First, spray with PB Blaster and wait 24 hrs. I can't tell you how many times this first step has saved the day.
Next, warm the aluminum using low heat. Careful here, you don't want the aluminum to get above 1000 degrees. In fact, a couple hundred is all you need. The aluminum will expand at twice the rate of the steel bolt. Just be very careful not to go nuts, warming is really all you need.
Get a purchase on the hex any way you can. The extracting socket seems like a good bet, but sometimes tapping a slightly smaller (six point) socket can work.
Careful when (if) it breaks loose, you don't want to destroy the surrounding engine fins etc..
I plan to die young, as late as possible.