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.
Try to predict Lex's auto aim ability and deflect it! This should be able to take Lex down to a low amount of health. If you aren't able to counter ti it will all come down to who has better aim.