Iris calls Barry for help while she is being shot at in an empty building in episode 2x03 "Family of Rogues".
Cisco and Caitlin disguise themselves as their Earth-2 doppelgängers to stop Black Siren from destroying several buildings downtown in episode 2x22 "Invincible".
Julian Albert tracks the monster's creator into an empty building and the Flash arrives just in time to stop Julian from shooting the kid in episode 3x05 "Monster".
Oliver and the other assassins arrive in Starling City and land on a rooftop in episode 3x21 "Al Sah-Him".
Maseo tries to get Oliver to snipe Tommy from a rooftop in Hong Kong in episode 3x02 "Sara".
The mysterious archer kills Tom Weston in the hospital in episode 3x02 "Sara".
Amanda Waller briefs Maseo and Oliver on their mission to recover the Alpha in episode 3x10 "Left Behind".
Maseo sets up his sniper rifle while Oliver slides down his zip line to the army chemical lab in episode 3x10 "Left Behind".
The Vigilante is on a rooftop across the street from City Hall scouting the building when Prometheus attacks. Later the Vigilante is setting up to kill Oliver during his press conference when the rest of the team shows up to shop the assassination in episode 5x15 "Fighting Fire with Fire".
From episode 3x02 "Sara".
Rip Hunter briefs his kidnapped team members about what he wants their help with in pilot episode "Pilot, Part 1". Jax returns from the future and visits Stein the day before they depart to request help in returning to the future in episode 1x15 "Destiny".
Vandal Savage brings the device from the A.T.O.M. suit and orders his scientists to reverse-engineer it in pilot episode "Pilot, Part 2".
Kara and Rhea meet to fight a single combat for the fate of Earth in episode 2x22 "Nevertheless, She Persisted".
J'onn talks to Kara as they watch the ceremony from a nearby rooftop in episode 3x01 "Girl of Steel".
Source: Melissa Benoist — Instagram
Oliver gets a call from Felicity about the wedding while he is fighting a group of ninjas in episode 3x08 "Crisis on Earth-X, Part 1".