The real counter to Tracer isn't any specific character, but understanding how her character "works". This is easier said than done, but I think I've mostly gotten the hang of it now, so killing Tracers are a lot easier. Instead of trying to fight them when they show up, you just let them do whatever it is they think they are doing, and then you turn around and point at the direction the Tracer came from, count to 2 and start shooting. 8/10 times you'll get at least a few body shots in before she tries to run off, it might be enough to kill her, if you get a headshot though, you'll definitely kill her.