Use a breaker bar and slip a nice 2' or 3' piece of black iron pipe over the breaker bar handle to lengthen it and get some additional leverage. I've used a pipe from my pipe clamp collection in the past (I also do woodworking). This worked great for the crank pulley on my '85 Volvo 240 when I had to pull the crank pulley to change the timing belt. You'd be surprised the leverage you can get by adding a couple of feet to a breaker bar! You can also sometimes "borrow" the handle from a hydraulic floor jack if you don't have any pipe handy.

If I couldn't loosen it with 3' or 4' of breaker bar + pipe, I'd put some gentle heat on it. Personally I use a propane torch but I'm VERY careful about using it near the engine compartment. Watch for rubber hoses, CV boots, and fuel lines! I would just play the tip of the flame over the bolt for maybe 10 seconds before trying to loosen it again. Sometimes the differential expansion between different metals (aluminum tranny case and steel bolt) can work for you.

One last point, and please don't be insulted by this because it's bit me in the past, more than once. Be sure you're loosening the bolt and not tightening it! Remember, lefty-loosey, righty-tighty!

Good luck, and please be careful if you use the torch (maybe a borrowed heat gun would be a better idea!).

Lou