How many friends do you have? Perhaps it would help to take multiple runs up the hill with only one or two passengers each time.
Definitely turn off the air conditioning.
My first reaction was to say that you should be in first gear all the way, since that’s the lowest forward gear ratio. However, it sounds like you might be building more speed than possible in first gear on the flatter sections and using that momentum to help get up the steeper sections. If so, then that could be the better way after all, although I’m pretty sure you don’t need to go past second gear for that.
In some cars, reverse is the lowest gear of all, but I suspect you’re not terribly interested in trying to navigate switchbacks on a mountain road going backward!