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When in battle you need to heal your teammates , use guardian angel as often as possible to become a more difficult target
If you use your Ultimate and find the enemy team crowiding behind a Reindhart shield, lock on to them all and go for it, the damage of the first shot will break the shield while the rest of the shots will kill the rest of the targets
Faster heroes like Tracer or Genji should avoid using the teleporter unless absolutely necessary (like overtime) to allow slower heroes to utilize the limited slots the teleporter has available.
Whilst his Peacemaker suffers from damage drop off at range, don't be afraid to take some left click pot shots at fleeing enemies
If you have two blinks available and an enemy shooting at you, you can blink once to go through him, and immediately blink backwards. While he turns around to find you, you get free shots.
Symmetra is a great counterpick against Tracer. Her shields make squishies harder to take down; her teleporter reduces the impact of Tracer's assassinations and lastly Symmetra is great at protecting flanks by setting up 'death rooms' with her turrets, with which she can seriously hamper or even kill Tracer.
After dying as Lucio, pay attention to the game feed if some of your team mates are dying, wait for them to respawn and get them back to the fight fast with your speed boost.
Mei's ice wall can protect you and your team from D.Va's self destruct. Just set it up between you and her mech.
Don't forget the melee strike when you hook someone. Lots of characters will survive if you just shoot them.