London. Enough said. In a situation like that, it's cool to nuke stuff if you're trying to clear immediate threats from an objective. The problem comes with nuking when there is absolutely no threat. Nuking unfortunately will never be enough on some maps, and some enemies will always camp the objectives. I quit in situations like that.
Yeah, I must have used up a good 3-4 missiles for Device 2 alone. The Phantoms kept spawning in so I had to keep my distance somewhat to avoid being sync-killed but that didn't stop the Alases/Turrets from staggering the disarmer x_x Literally as soon as I finished nuking 3-4 enemies to provide breathing time, it seemed like the next group of spawns would immediately converge. This issue kept rinsing and repeating over the course of four minutes.
There is one more important thing you missed. Spawn spot suppression. The upper level bridge is a spawn spot. Before nuking the atlas, you need to run up there and suppress it. This pushes the spawn to the next available location and buys the disarmer time. Its best you protect the disarmer as much as possible through drawing aggro or missiling before the player can be pushed off the device because you have to remember that every second means the spawn is converging on you.
As the disarmer most times, its a few things that causes failure.
i) No one bothers to cover you. This is the dumbest move available and says much about the team
ii) Your team covers you in the wrong manner. Standing next to you is the most common one.
iii) No one is experienced enough to know which spots are tough and should be done ASAP. This leads to them running away to recover shields instead of standing to defend you.
iv) The disarmer starts and stop because the player is afraid of dying because their shield broke. This is incredibly dumb. Do it or let someone else do it but do not do it halfway. You are just screwing all those who expended missiles, ops packs, etc to hold the line.
Because most pugs suck for teamwork, its mostly best to let them all die then kite and do it yourself. The important thing to remember is that you have time. A device takes maybe 10 seconds to reach and 10 seconds to do so if the atlases are slow, let them take their time. Prepare a missile to clear the device area and hope for the best. For london, lead them to the red car. The amount of obstruction from there to the middle device helps prevent hits. Also make sure the half cover at the device is between you and the red car.
Oh and if you are covering the fella, you can draw aggro from the red car's direction by simply going into hardcover at the half cover near the ammo box. The atlases and stuff will shoot you to no effect so just let them shoot. Respond only when dangerous enemies close in on you (eg phantoms and dragoons). One guy can draw the aggro from the LZ zone by going into hard cover at the junked makos near the ammo box opposite the bridge.
Its best you protect the disarmer as much as possible through drawing aggro or missiling before the player can be pushed off the device because you have to remember that every second means the spawn is converging on you.
The other problem we had was the risk of getting sync-killed by multiple Phantoms. I kept trying to stagger the Phantoms, planting the Atlases in the bubble and wearing down their defenses hoping to draw fire.
As the disarmer most times, its a few things that causes failure.
i) No one bothers to cover you. This is the dumbest move available and says much about the team
One person was constantly trying to disable while the other was trying to clear out the spot. Due to the amount of constant rockets/staggering we kept going down. This is when I (or a teammate) would attempt to missile the surrounding area until I realized the game was just re-spawning a new group of enemies to converge where the last group was annihilated and then thought "Hey maybe we should run and kite them away."
Then we realized that meant no one could do the objective due to damage radius.
ii) Your team covers you in the wrong manner. Standing next to you is the most common one.
Tried kiting the enemies away. That didn't work for the aforementioned reasons - it's not always possible to dodge a rocket/Turret on London while retreating behind a column/wall/staircase on London.
iii) No one is experienced enough to know which spots are tough and should be done ASAP. This leads to them running away to recover shields instead of standing to defend you.
Fair enough. I'm still learning this aspect of the game.
iv) The disarmer starts and stop because the player is afraid of dying because their shield broke. This is incredibly dumb. Do it or let someone else do it but do not do it halfway. You are just screwing all those who expended missiles, ops packs, etc to hold the line.
Hm, well on Silver I can understand how this would seem foolish, but on Gold (unless you are a tanky character) your shields often feel like tissue paper. If your shield just got wiped and there are at least 2 other boss characters in a wide open space, in my experience I've found the likelihood of not being incapped is extremely low.
I even tried disabling the device myself letting my shields/health get wiped entirely (as did my friend), hoping the aggro would be drawn away long enough for some breathing space. Unfortunately it didn't work.
(I think I started that wave with almost full consumables and ended up having 1 missile, 1 Shield, and 0 Medi-gels left with all my teammates dying because we ran out.)
A device takes maybe 10 seconds to reach and 10 seconds to do so if the atlases are slow, let them take their time. Prepare a missile to clear the device area and hope for the best. For london, lead them to the red car. The amount of obstruction from there to the middle device helps prevent hits. Also make sure the half cover at the device is between you and the red car.
The Atlases are very slow but their rockets aren't and often have DOT radius. Regardless of that, next time I'll keep on the lookout for the red car. 
But this: Spawn spot suppression. The upper level bridge is a spawn spot. Before nuking the atlas, you need to run up there and suppress it.
I really wish we had known this back then. Would have helped immensely. Man, I thought the 1234 and Escort missions on Rio against Collectors was bad. But that particular London match was the worst I've seen yet. The game really gave us the finger on that one.