So i played a little of sombra today, and i realised: She's the perfect mercy counter!
Here's what you need to do to counter mercy in three easy steps:
Step 1: get good at aiming with Sombras SMG
Step 2: Punish Mercy when she resses without her ultimate active
Step 3: As soon as she ults, hack her or use your EMP when under fire
Step 4: Enjoy!