I usually sneak behind her, look at the right timing when she is sniping other member so she will be clueless then TP behind and charge orb + beam combo or turret plus Orbs combo.
symmetra can pretty much go where widow can access so shell never see you coming, thats the good thing about symm 3.0, no one knows or forgets that she can do it, widows will often see her as a last priority most of the time.