Instead of nerfing Mercy just increase her Skill-Cell. This changes will make mercy a lot more powerfull but she will shut-up the mouth of these players saying she is a BRAIN-DEAD hero.
First adjustment HEALING-BEAM
A) Add ammo to her healing/power beam and increase her healing per second to 66
B) Force Mercy to aim similar to Moira increasing her healing per second 72
Second adjustment GUARDIAN-ANGEL
A) Increase guardian angel cooldown 2 to 3 seconds and increase the speed 20 to 30 meters per second
B) Reduce guardian angel range 30 to 20 meters
Third adjustment VALK-ULT
A) Valk will fuse Power/Healng beam the entire ult duration but mercy ult generation ratio will be reduced 10%.
Four adjustment REZ
A) Mercy will be able to option “INSTANT-REZ” but she will kill herself on the process with a 12 seconds respawn. If you wait the 1.75 seconds to REZ somebody the ability will work in the same way as that is LIVE
Last adjustment BUG-FIX
A) Mercy can continue healing 1.5 seconds after she lost the sight with her target, fix this bug. She will stop of healing her target instantly if she loses line of sight