How do I beat Uprising on Legendary difficulty?

From a guide on /r/overwatch written by /u/Prof_EobardThawne:

Phase 1 (HACK #1):

This is obviously a pretty easy one, even for Legendary mode. Don't dick around is the biggest part to beating Phase 1, but other than that there's a few things to speed it up.

• Tracer: Usually positioned on right staircase inside/outside the garage. Shouldn't need to be healed too much here, as you're behind the spawned bots.

• Torbjorn: Sets his Turret up immediately to the right of the Terminal's position. From there it can watch both the immediate threat of the back stairs, but also the front if any gets past Reinhardt and Tracer.

• Reinhardt: Mercy will be doing most of your damage, don't get too greedy and move past the garage doors, as you'll be brought down pretty low pretty fast.

• Mercy: Again, you will be most of the damage coming from the front doors, but don't forget to damage boost Firestrike.

Phase 2 (HACK #2):

This one is a step up in difficulty. The biggest thing would be to get that Turret down and upgraded ASAP. Don't be afraid to hide inside buildings if Reinhardt needs to recharge his shield. You have unlimited time, but the longer you wait the more that will spawn.

• Tracer: The best spot we found for Tracer was right in front of the Cathedral doors. From here, she can shoot off the ShieldBot's shield arm and can otherwise do a decent chunk of the damage. If you need healing, obviously fall back to the Terminal.
• Torbjorn: Best place for your Turret here is immediately in front of Terminal. Get Armor for Mercy and Tracer and otherwise deal with the bots that will spawn immediately behind the Terminal / statue.
• Reinhardt: Shield-duty. Soak up as much damage as possible, and do not be afraid to Earthshatter AFTER the ShieldBot's shields are down. Your Earthshatter is blocked by the shield just like any other enemy.
• Mercy: You won't be doing much damage here besides clearing away Enemies who are behind your Reinhardt and Torbjorn. Save Resurrect for Reinhardt, or both of the DPS. Usually will need it here at least once, depending on how well Tracer does.

Phase 3 (HACK #3):

DO NOT START this Terminal until you've killed all previous Phase enemies. It makes it that much harder when you step on the Terminal and the new enemies start spawning.

• Tracer: Best position is the right staircase. Usually our Tracer ran behind the ShieldBots and took out their shields first and foremost. Don't be afraid to Pulse Bomb off cooldown on the ShieldBots.
• Torbjorn: Our Torbjorn put his Turret on the top left balcony. You can find some boxes near the back to get up there. From this angle, it can cover front entrance and the left side entrance. Armor, Armor, Armor.
• Reinhardt: Our Reinhardt held the front left entrance. Be careful of Line of Sight here, as the Mercy will be the only one on the Terminal, and she cannot step off. Feel free to charge in the front entrance all you'd like, but be wary of your health and positioning -- otherwise Mercy won't be able to heal you.
• Mercy: You will be the one on the Terminal this Phase, so your biggest goal is to use the cover that's provided for you, and make sure to use damage boost on Reinhardt, as he'll really be the only one you can see. Resurrect will almost certainly be used here again, but be careful and don't be too eager to use it. If you can use Reinhardt's shield to cover and do a manual revive, then do so and save your Resurrect.

Phase 4 (PAYLOAD #1):

This consists of getting the PayLoad up and running. Pretty straight forward, but probably the hardest Phase. Your biggest concern here is the Bastion drops. Your team's focus should be on the Bastions 100% once they spawn. Your Ults should have enough time to recharge between the Bastion spawns, so make sure to use them -- but be quick about filling it up again. Bastion spawns start when there's 2 Minutes remaining, so other than them, make sure to kill the LaserBots ASAP. They'll destroy your Payload health faster than most every other enemy. BombBots are relatively easy to deal with. Don't bother bursting them down or using Ults on them. Just have your Reinhardt Charge them right as it starts to Self-Destruct. It will take maybe 25% of Reinhardt's health, but if you let it Self-Destruct on the Payload, then it will take 25% of the Payload's -- not good.

A good Payload Health to begin the next Phase should be around 40-60%. Any less and it makes the next Phase that much harder.

• Tracer: You'll mostly be running between the right entrance (archway) and the left entrance (statue). Pulse Bombs can be used at your discretion, but make sure to ALWAYS have one for the Bastion. Mercy will be primarily damage boosting you, so be sure to stay in her Line of Sight.
• Torbjorn: Before everyone gets on the Payload and begins this Phase, go upstairs and set your Turret up on the left entrance balcony (statue side). You won't be with your Turret to repair it much, so be sure to keep an eye on its health and use your Ult if necessary to keep it alive. Assist Reinhardt on the left entrance.
• Reinhardt: You'll mostly be covering the left entrance as said - but be careful of positioning here again. You shouldn't be using charge until BombBots start spawning. Use it immediately when they start to Self-Destruct. Otherwise, let your team do the damage -- keep your shield up and protecting both your team AND the Payload as much as possible.
• Mercy: You'll be damage boosting on the side here, but make sure to use your side-arm as much as you can. Headshots do wonders here. Resurrect is going to be almost mandatory here, so be wary of when you use it -- not too early.

Phase 4 (PAYLOAD #2):

This Phase is a step-down in difficulty, but still hard none-the-less. Bastion spawns aren't as frequent here, but they are in a permanent Tank form. They usually do focus the Payload, unless you get pretty close - so keep your Shield up and keep your distance. Think of this as moving the Payload in any Competitive game -- stick together (That means you, Tracer).

• Tracer: As I said earlier, don't stray too far away from your Shield here. The Bastion Tanks will 1-shot you if it's a direct hit (also, they're Aim-Botting -- stay behind the Shield).
• Torbjorn: Set your Turret up on-top of the Payload, but don't expect it to do too much here, as the health on most of the enemies in this Phase is usually much higher. Get that armor down and focus on the side-enemies.
• Reinhardt: Your Shield here is of utmost importance. Not only for protecting yourself and your teammates, but also because it blocks damage from the Bastion Tanks hitting the Payload. You will have to Charge a few BombBots here, so be on the look-out for them.
• Mercy: You will have to swap from doing damage and healing frequently on this Phase, so always be maximizing that down-time (Don't reload, just heal someone and it will auto-fill.) Resurrect may be able to be saved here, but better to use it than not to.

Phase 5 (PAYLOAD #3):

Final Phase, one of the harder Phases solely because you are now TIMED. You have 5 Minutes to kill every single Orisa. Bastions are frequent and will spawn infinitely until you kill all Orisas. There is no hiding around corners beyond the brief Shield recharge, so make sure to keep an eye on the clock. There's also an abundance of crates to use as cover, both for yourself and for the enemy to use. Take advantage of peaking corners especially for those Bastions. Enemies will spawn infinitely here until the x4 Orisas are taken care of. They themselves don't do too much damage, but they constantly throw down shields and throw your team around.

A valid strategy (not the one we used) is to bum-rush the Orisa. They spawn in 3 different waves, the farther in you get, the more spawn. Don't have more than two at a time, or you'll never even break the shields.

• Tracer: Another harder one for you, as the Bastions will chew through your health in under a second UNLESS you have Armor on. Armor reduces the effectiveness of normal bullets by a large margin, so make sure Torbjorn is prioritizing you and Mercy for Armor.
• Torbjorn: Probably the hardest on you, solely because you won't have a place to just set your Turret. You will have to constantly be replanting it, and don't expect it to last long, or even get it to level 2. Armor is a game changer here, mostly for those Bastion turrets. Don't be afraid to Molten Core without a Turret up, whether it be defensively or offensively.
• Reinhardt: Shield is another big one, but don't let the Bastions chew through it just to get a few bullets of from your DPS -- use cover. Charge only for Bastions and Orisas, but don't move out of Mercy's range.
• Mercy: Won't be doing too much damage here, besides the few little bots that spawn behind you. Focus on healing Reinhardt and your Tracer, but also be damage boosting at all possible times, specifically when a Tracer is downing a Bastion turret.

Final Notes:

That turned out a lot longer than intended, but other than those major concepts, there's a few things you can do to help that take a bit more time, but are sometimes worth it.

• Tracer can Blink behind ShieldBots and shoot the left arm. This causes their Shield to break. Reinhardt can also Charge and it will instantly break it.
• You can wait before some of the next Phases and let your Torbjorn get some Armor down before you start. Highly recommended.
• You can see some of the Bots being airlifted down, if you shoot the little capsule they come in before they begin landing, you will destroy it instantly. Works well for your Mercy to do this on the side.
• Damage doesn't interrupt the manual Revive.
• Either Molten Core or Reinhardt's Shield can be used to do a manual Revive. The former boosts your health tenfold, so you can Revive without worrying of dying.