Symmetra can put up to 6 turrets on the field at a time, but she can only start with 3. So pick a spot during a defend map (easiest since you get setup time). Throw 3 turrets behind a wall, then wait til the cooldown lets you place another. Repeat this until you have six and be bait luring in Reapers, DVAs and others that will charge your capture point you are defending.
The 7th beam? Its Symmetras own gun so fire on them as your beams fire on the enemy and you get the trophy.