If you wanna attack a boa of good size you're gonna wanna attack the tail first. Grab with both hands get low and drag him backwards as he tries to snap/latch. Do this for at least 20 yards always circling away from the head. As he slows you want to throw a towel or shirt over his head. Secure with both hands directly behind their jawbone and stradle as far back with your knees with base and squeeze. This prevents coiling, which intern prevents constricting.
If he bites you immediately push down and forward hard right behind his jaw then up and back. You have to rip the bottom teeth through your flesh so you can start to fight the coils. They're already happening, so address the bite, then the coils, and you'll be fine.
Trust me. I'm an astronaught.