As ahri, In late game team fights, you can either look to flank and pick someone off with your allies. This is a good way to play in the late game if you’re ahead. Just make sure you save 1 Ultimate  stack to escape.

Alternatively, you can peel for your carries and keep them alive: using your E on the closest target to your ADC and peeling for them.

The fog of war is your friend and you should use this to set up death brushes with your team. This works especially well when a major fight is going to take place for a neutral objective like the Baron. Also, try to take fights in closed areas like the Jungle, as it will let you hit multiple people with your Q. As ahri, In the team fights at the end of the game, you can either look to flank and choose someone with your allies. It's a good way to play in the game at the end of the game if you're in advance. Just make sure you save 1 Ultimate pile to escape. Alternatively, you can peel for your carts and keep them alive: use your E on the closest target to your ADC and peel for them. The war fog is your friend and you should use this to set up death brushes with your team. This works especially well when a major fight is going to take place for a neutral goal like the Baron.