Mercy is the generic healer of Overwatch's support group and her targeted healing can keep friendly heroes standing against even the toughest of assaults. Fortunately, Mercy has some great tools to keep her in in play and her ability to resurrect recently fallen heroes can turn the tide in a match. If you love dishing out big heals or saving the day with the press of a button, Mercy is your pick.
Caduceus Staff [LMB/RMB] - Mercy's staff has two functions, the first being a healing beam that quickly recharges the health of your allies. The RMB beam increases their damage, and switching between the two is very quick so you'll want to get used to flipping back and forth. Unfortunately, Mercy's Caduceus Staff can be a bit unwieldy when trying to heal a damaged hero when others are crowding around as its generous auto aim will often select the wrong target.
Caduceus Blaster  - Next to Torbjorn, Mercy is the only hero with a secondary weapon. Her Blaster isn't going to send the enemy running, but if she's attacked without backup it can be pretty effective at scaring off fragile heroes. It's pretty unremarkable though, so I'll forgive you if you forget it even exists.
Guardian Angel [LSHIFT] - A big reason for Mercy's efficiency as a healer is her ability to quickly fly to any hero within range. Guardian Angel is a bit floaty, so it'll take a few tries before you're using it gracefully, but the cooldown is so low that you can spam it frequently to save a teammate's life or flee to a black-line hero if things get sketchy.
Resurrect [Q] - The holy grail of game saving ultimates, Resurrect can bring an entire wiped team back from the dead if used properly. Casting it has a short delay, but each hero will be brought back at full health. There's a 15 meter range, so positioning yourself properly can be an issue.
Angelic Descent [Space] - Pressing spacebar while Mercy is airborne lets her glide gracefully downward. You won't get a ton of opportunities to use this, but it can be effective if you use Guardian Angel on an airborne Pharah, letting you damage boost her attacks until you fall back out of range.
Mercy Strategy Tips
Unlike other supports, Mercy has a singular purpose: keeping heroes alive. The healing beam from her Caduceus Staff is strong enough to heal through most attacks, so if your tank comes under heavy fire you'll want to be right behind them keeping them alive.
Being an effective Mercy also requires having a good understanding of other heroes (good thing you're reading this guide) because certain teams will be more reliant on healing than others. With a resilient team, Mercy can spend much more of her time shadowing high-damage heroes like McCree, Widowmaker, or Roadhog using her damage boost to make them even more lethal. When a hero starts taking too many hits, use Guardian Angel to zip over and heal them up.
A big problem with Mercy that many players have is an unwillingness to use her ultimate, Resurrect, due to fear that other heroes will die shortly after. In some cases that can be a smart decision—especially during the final push of a map—but in a lot of instances it's worth it to bring back even one or two heroes. That's because Mercy's healing beam will recharge her ultimate meter extremely fast, so go nuts with resurrecting.
Other than that, avoid the tendency to heal targets that simply don't need it or are already full. There's a difference between actively healing a target under fire and just wasting precious seconds pointing the beam at someone. While healing is crucial, a Mercy who knows how to balance her team's health while giving damage boosts will always be more effective than a Mercy who just runs around healing everyone and nothing else.