I just played a comp game were the enemy widow constantly was impossible to see or even hit.
She was most likely a cheater as well, due to the fact she barely missed any shots, but that made it the game worst as she constantly hid and was constantly invisible / hiding behind walls or above.
Widow maker is literally to hard to see on most maps, if not every map, especially with the skins see has.
Please make her visible on all maps and maybe make her damage get reduced as she mainly on hits way too much (and its getting very annoying).
maybe switch to something that can sneak up on her then or get you vision checked… 
she is not that hard to spot… but she is hardly going to stand right out in the middle of the map waving a orage flag wearing a bulls eye hat or something…
my suggestion is to be behind the widow maker not in front of her… that is where the shooty end is… you don’t wanna be in front of the shooty end… 
try shooting or sneaking her when a mercy is ****ng her up with damge and heal.
mercies rarely do since it makes more sense to pocket other characters… notably ashe who otherwise cant one shoot…
Also Mercy is one of the easy ways to take out a widow maker at least at lower ranks.
Again don’t be in front of the shooty end…
Also Pharah is way worse when pocketed… also more valuable to pocket since her and bastion and ashe are the few that really do more damage then mercys own pew pew gun would do…
So for pure damage that is what I prioritize when I play mercy… so widow is not high on my radar… unless she is already on fire and giving good value then it may be worth it just to ensure she secures those picks and make the fight easier to win for the rest…
anyway what where you trying to use when you had such trouble with widow makers?
I mean some heros are just a bit to all over the place to spot things…
Genji, Tracer, Junkrat can be a bit like that too… then again those more chaotic chraracters usually don’t get picked of my most widow makers that often… since again… pure chaos… oh and doom fist…
though again I suppose junkrat can get headshot a lot… but half the time he randomly mines himself mid air and totally changes vetor to who know where… if junkrat does not know where is going to end up how can anyone else lol 