Mercy is hard as fuck to play.
How so? Heal people when they are wounded, if noone is wounded damage boost those that are dealing damage consistently or using an offense ult, follow some heavy pusher around and heal them consistently to further the push... When you have really nothing to do or/and are harassed whip up the gun and shoot a bit.
Use your ultimate to revive 1 guy if that 1 guy mean that your team will avoid being wiped, otherwise wait for 2-3 guys to be dropped before using, and try to not get focused and die before you can use your ultimate to extremely boost your team push potential.
Also spam the wing-boost ability and floating capacity whenever you can to reach your team faster and stay in the air (harder to hit).
Cant really think of anything else and I felt like I "mastered" it too quick, missing something or I really dont see how Mercy is hard.
Maybe I'm just a born healer? But thats a bit of a problem 'cause I like to see myself playing offense badass characters