This guide is one of the ways I found that makes this challenge quite easy. I will be outlining hero roles/locations and some tips per phase in the run. The wheels in this game mode are like normal wheels, same HP and speed. They are dangerous. (TLDR at the bottom of the post)
The main cause of defeat in this game mode is actually related to positioning. It is vital for every piece to be at certain locations to minimize risk as much as possible.
At the time of writing this guide, I have only been successful with this lineup, after trying some certain configurations up.
Torb’s role is the most important role. He is the guy that pulls the bosses aggro. This concept is important, because every time a boss dies, they always drop two wheels at the door. He is also a must pick because of his ultimate ability. His position is the center. The turret will be placed either on the ramparts, or the high ground to the right.
Soldier’s role is mostly taking care of the creeps and using the burst of his rocket to eliminate the wheels. You may use his ult liberally and there is not much reason to hold onto it. His position in the map is mostly up on the ramparts (top left side).
McCree’s role in this, is his ability to burst down bosses as usual, but there will be certain guidelines with his ult usage. He is also good at bursting down wheels with his fan the hammer. His position is center, near the door.
I initially doubted this pick, because of his ultimate not being very useful, unless in very situational cases. He is good however because of his ability to heal people without paying attention to them, and especially the discord orb. His position is mostly to roam and try to pop bots/wheels before they get too close to the door.
Phase I – Spawn/General guideline
DO NOT attack boss until it aggroes on Torbjorn, so he may aggro the boss away from the door successfully.
Torbjorn may save ultimate to make a lava path for when boss is about to die.
Don’t die (especially Zenyatta)
Throw turret in designated position
Hold center bridge, focusing also on right lane.
Toss lava onto boss before it dies.
Pre-jack boss spawns to aggro easier.
Phase II – Reaper Spawn
Phase III – Monster
Ult on Monster
Try not to get hooked
Phase IV – Dragon
Phase V – Junkenstein
Hold center again
Shoot 2 or so globs up where Junkenstein is (pointed out by u/aa728 )
Fire globs at common wheel pathways.
General note for Phase V
Phase VI – Witch (MOST IMPORTANT PART)
Head down to center focus on creeps and wheels.
Use ultimate as soon as the fight starts, clear as many creeps as possible, don’t focus on bosses.
Save rocket for when Witch and Junkenstein die.
Highly suggested to go up the ramparts for better vantage point for ult, and also to avoid aggro/ draw aggro of Junkenstein away from your team.
Make sure for Monster hook to be ON cooldown, or else he may cancel your ultimate.
Make sure to kill only the Witch and Junkenstein with your ultimate (prioritize the doctor). This is important that way the number of wheels that will spawn will be manageable.
Two ways to selectively kill: Don’t let Monster get full duration of your ult (optimal) OR put the Monster in some blind spot of your vision.
Save ultimate for when McCree pops his ult
Pull aggro to keep Witch and Junkenstein near each other.
Make sure the spawned tires go the same path, so your ultimate may be better.
Soldier kills bots and ignores bosses. Holds ramparts.
McCree selectively kills bosses (Witch and Junkenstein) in last phase. Holds door.
Torbjorn pulls boss aggro away from the door.
Zenyatta uses ult sparingly. Holds right and center.
That’s all and good luck.