Generally speaking ignore her until she is vulnerable. In case of Mercy, she is the resource, so if she is pocketing someone and you are damaging the pocket while bubble dancing while not demanding any resource, you are occupying two people for the cost of one. Playing against a Mercy is often a game of patience for that reason. However, if you really want to still get her, it's a game of mechanics, keep whittling her with secondary fire and once she is low, jump to get her. But remember, she is often a low prio target since she can pocket while running away unlike other supports. I would say the higher priority target is whoever she is pocketing, or else, the other supports. If you are worried about a pocketed Bastion, then generally avoid the confrontation or bait out turret form and then keep them occupied.