preemptive cc.... preferably hard cc....
to give u a successful example from a game of mine including hypothetical....
(im bottom) we got the enemy team to go in the choke pt from baron going down (south) after a good baron fake bait.... im waiting in the second bush down from the pt... already checked for wards, its me and the under fed adc, the adc leads the bait from baron down to me (sona) in 2nd bush, diana does her crescent thingy, best is to go to her and avoid it (or wherever) but if not, like my adc.... keep moving to the trap..... she hits the adc, as soon as the adc is hit according to your reaction time, u slam that hard cc down... sona's ulti in this situation...
aka....
diana crescent....u see adc is going to get hit... u hit ur cc (hard cc pref), begin damage rotation... adc should begin damage rotation at this point, orb walking if possible to bush/away from danger pt.... the support moves out of protection/bush/where the adc is going an create a secondary target for diana to focus on, as well as any follow up enemys who will be coming to this point.... finish her off, possibly flash to safely if the enmy's were close enough, otherwise move to tower recall for wards ect, ect...