In team fights you want to focus their AP or AD carry. You should decide which one to focus based on how fed they are and how easy they can escape from you. If your team is ahead and have good follow up you can simply initiate with your ultimate and then E, pulling the enemy team in closer, and then popping your Zhonya's active when they turn to focus you. Otherwise, you want to wait for your tanks to engage and then try to land your Q on the enemy AP or AD carry. It's extremely important to land your Q in team team fights as it's your only form of gap close and without it you'll just be kited to your death.