Pressure washing seems like a good idea to me to get as much loose paint off as possible, though I've never tried it.
To remove any paint remaining after pressure washing by hand scrapping, DO NOT use a heat gun or propane torch to soften the paint as you scrape--that is unless you're fully insured for fire and want to collect on your policy.
If you have a lot of scrapping to do, you can substantially cut your time with a Porter Cable power paint remover (model 7403). It sells for about $225. It resembles a router but instead of a bit, has a 6" tungsten carbide disc (24, 36 or 46 grit) which, unlike a grinder, rides flat on the surface where you want to remove paint. A guide will follow the exposed shingle (or clapboard) edge.
Before repainting, consider why exisiting paint is "barely hanging on"--unless you want to repeat your exercise in a few years. Consult with a number of paint "experts" in your area. Is excess moisture from inside your home the problem or is it external? Was a high qualitiy oil paint used? Is ultra violet sunlight destroying the protective quality?
I formerly manufactured cedar outdoor furniture which sat it the weather and sun year round. Cedar is an oily wood, the oils are what gives it it's natural resistance to decay. Eventually (in 60 years certainly) these oils lessen or disappear leaving only the surface paint coating for protection. Heat, cold and humidity causes wood to expand and contract. When it does, micro cracks open (which soon become larger visible cracks) allowing moisture in the form of humidity or rain to enter and soak into the wood fiber. Once there it can't exit via the crack. To get out it pops the paint loose, compounding the problem, as more paint pops lose as more moisture soaks into the wood fiber.
Ask your paint/stain dealers about a product that will soak into the wood fiber, waterproofing it and provide surface ultraviolet protection in the color you want. Thompson's. makers of Water Seal, have such a product as do other manufacturers.