Movement is key! use your dashes to move around him. If you're running low, Reflect so he'll stop shooting. it'll take a little for his gun to start back up, capitalize on that time. When ruckus ults, just deflect it. he usually wont stop.
Ruckus is a good champion to use "Reversal" on; it might even outright kill him when he's using his ultimate.
Try to get in front of him quickly when he's already firing on your teammates, but be sure to have an equally fast escape!