tracer is a loving nightmare to deal with, mainly because you can never hit her without a good roadhog or mcree
fighting a tracer just requires you to dodge and then attack her at the right time, force her into using her recall early and try to make her use all 3 of her blinks and she can't do much to avoid you any more.
her damage is very low unless she gets right in your face, which you can just blast her down pretty quickly anyways
If all else fails, just get a pharah.