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When you play a symmetra make sure you flank as you can take out people that are alone. ;)
stand in a position that benefits a bastion, don't worry about Genji or anyone else. They will be rushing forward as soon as the game starts to ult charge
Use your blinks to escape first, before using recall. It should be used as a last resort.
If an enemy Tracer, or genji's deflect stick a pulse bomb onto you, be sure to recall right before it explodes, as you are invincible while recalling and it will drop the pulse bomb on the floor and hopefully will kill the one that stuck it on to you.
Use the biotic nade mostly offensively, you can heal your teammates with your gun and can get gealth from other teammates and health packs. Removing the enemy's ability to heal themselves is huge.
Contrary to populae belief, ana is VERY strong against flankers if you have good aim. Sleeping a flanker will remove their value from the game for 5 seconds and will allow you to get to your teammates, who won't like a genji inside their deathball.
You can build up speed as Mercy by jumping just as she lands after using Guardian Angel.
If you press E (cannons? missiles? idk what they're called) you can put up the defense matrix for protection + damage dealt.
Cannot stress enough to do the freeze-headshot combo
There's an epic Reinhardt combo out there: Earthshatter, Fire Strike, then Charge. Basically, it's a stun, damage deal, and instakill. Very fun to hear the reactions of people when it happens ;)