@Diddums, I think you should stick to what you do best and more importantly what feels comfortable to you.
I don't see many attacking Mercys about tbh. @tronic44 got an amazing POTG by killing 2 people with Mercy...yeah, it was funny but thinking about it we were steamrolling the team.
Her gun is as powerful as D.Va's pistol, however the fire rate isn't as good so I find it tricky to switch when I have played Mercy previously.
One main thing though. Mercy staying alive is key. That's way more of a focus than trying to shoot a gun.