I disagree with point 1, having the team spread out is a great way to get your Mercy killed. Very often Mercy will have to fly past a door way and that guarantees she'll be headshot by a widow maker. It's much better to just protect her. Listen to the audio queues, if you hear her say shes under attack, stop and turn around, you're likely being flanked. That's not to say bunch up too much though, if you do everyone is going to die to a hanzo bukakke shot.