The team I used (which worked on the first try) was composed by McCree, Zenyatta and Torbjorn (I used him). Basically:
- Torb should put his turret on the left flank (at the top of the stairs) and cover that area. His ultimate should be used whenever a boss appears, throwing some lava in the left flank spawn point and the rest on the front gate (with the exception of Junkrat spawn, in which case the lava designated for the front gate should be used againts him instead).
- Zenyatta should put a discord orb on zombardiers whenever there is no boss (otherwise priority #1 is the boss). Of course, he must keep an eye on his teammates whenever they need healing. Also he should save the ultimate only on bosses fights and whenever he is going to die (make sure to put discord orb on the boss beforehand). Also, he should be placed in the front gate killing enemies on the front (mainly) and right (from time to time).
- McCree he should go to the right side, on the elevated area, covering the right side and the center. His ultimate will be very crucial on boss spawns.
Last tip: don't waste ultimates on Reaper's third spawn, those should be saved for the Mercy + Junkrat + Roadhog spawn.