Movement:
As mercy you should constantly be utilizing Mercy's ability to fly towards other players in order to heal them on Xbox this is the LB button and on pc It's Left Shift. When a team-mate's health is lower or critical use this ability to fly towards the player and heal them, then immediately move to the next player. Do not stay in the same place for long periods of time, and constantly adapt your moving to the map & current scenario. Always go for the players with the lowest health or the players with a key power in the match i.e. if the match requires a Roadhog, but doesn't require a JunkRat heal the Roadhog first, then instantly move on to the Roadhog. As mercy you will have to constantly be moving, so practice flying between targets in Skirmish, the faster you can move to another player the better chance of getting the highest healing done. Remember Mercy automatically heals herself, so when traveling to another player you will gain most if not all your health back.

Healing / Improving Player Damage Output:
Healing is rather simple, hold RT or Left Click. Healing beams, will give the player health as long as the beam is traveling directly to the player. It is quite important to note, that you should buff the "Attack" class players with the RB or Right Click. This increases their attack output this is very important in leading your team to victory. Classes like Junkrat will require more healing than their do improvements in Damage Output, due to JunkRat being very powerful when used properly but being limited to 200 health, and being rather fragile.

Mercy's Ultimate:
Mercy's Ultimate revives one player, Mercy has a limited amount of time to travel to the player and get within range to use her Ultimate. Once her ultimate is used if within the time, the player will instantly respawn at the same location with full health, you should then heal and protect that player just incase there are any surrounding enemies remaining that killed the player in the first place. Always remember to revive specific players, usually the ones that are pushing their weight the most e.g. the ones with most kills. Do not revive players that aren't doing very well, as it will be a waste of an ultimate and could cost your team a win.
Overall:
Mercy is a very easy Hero to play, as long as you're very good at moving around the map, and healing players in the fastest time possible. Just remember to focus on healing and buffing the right players at the right time, and focus on the objective with as little sacrifices as possible, remember that your ultimate is very powerful and that it should be used very wisely depending on the current gameplay situation.



(All Written by me, If there are any errors or mistakes or if you would like me to change anything then let me know)

Hope my guide helps some people.