This mode features sped up zomnics and can quickly go downhill if your team gets in a bind. It's still the standard mode, so nothing else has changed.
I'd say Torbjorn is an absolute must for this mode, as his turret will help clear things out and more guns on your side is always a good thing. I think you can be pretty liberal with your use of his ultimate, as with the turret going strong, the meter will build fast. Throwing it down in front of the door is an okay tactic if a big mob is coming.
For healer, that will be personal choice, as they all have pros & cons. Zenyatta is probably the best & will save your team from death with his ultimate, but in this mode where speed is key, not being able to deal damage during the ult for a few seconds could be bad. Baptiste has good healing, but for maximum use of his alt, you basically have to be all collapsed back near the door. Ana works well with a good player.
For DPS, Soldier 76 & Ashe are pretty good picks. Soldiers ultimate is great for clearing out zomics if they're getting out of hand. A well placed BOB can cover & defend the door in a hairy situation. Hanzo has good crowd control with his ult, but you'll have to be pretty quick to pull off his headshots on the quicker zomnics. Mcree is useful for deadeye to clear out a crowd, but the player picking him will have to be pretty solid with hitting headshots. The zomnics are too fast for him to manage the stair route if he's only doing body shots.
Personally for me, I'd avoid Widowmaker, unless you're a total beast with headshots. Her ultimate is totally useless in this mode & venom mine isn't gonna do much damage & has a long cooldown.
As for bosses, try to take them down quick, but don't let the zomnic mob build up while going in 100% on the bosses, cause they will take the door down quick. For Symmetra part, I'd suggest everyone collapse back to the door, cause the zomnics will quickly get past her shield wall & any players behind it will be useless.