I have 3 techniques for shooting flying dragonflies. 1 - using point autofocus, the larger dragonflies tend to stop and hover for a moment making it easy to capture. They initially look like a dark smudge when autofocusing. 2 - using manual focus and firing off a burst while slowly adjusting the focus. I use this when they are on the move (or mating). I've used this on other flying insects too. 3 - observing their behaviour and taking a photo where you expect them to land. Dragonflies I've found will frequently keep landing back at the same spot when they are hunting. In this instance, I focused on the landing zone and observed it coming back to roost and fired a burst shutter.