This level must first be unlocked by interrogating the Infiltrator. There's a variety of conversation paths, but essentially there's a 'nice' end where you convince him to actively want to help take out Mosaic, a neutral end where he gives in and divulges the information, and an aggressive path where he tauntingly tells you what you need to know, expecting you to die trying. It doesn't affect gameplay, but it does change his dialogue for most of the rest of the game.
He will inform you that you'll need to steal a Phase Plotter from one of their Spire hangars. As Agent Nico DeSilva is currently MIA near a Spire in Pima, that's where you'll be going.
After being dropped off, listen to the dialogue. If you've missed/skipped getting the Laser Pistol via the Minor Operations, there's one in the hand of the toasted Grunt at the exploded car. For the Laser SMGs, there's a few weapon lockers in the tower whre you can get it. The first area of engagement is in 4 waves in front of a car dealership: a grunt and some Sectoids you can catch unaware, followed by a drop pod from down the road you must destroy to stop spawns. The third and fourth wave will come from the dealership itself.
Going through the dealership and it's mechanics bay, you can find the Tactician Pack schematics on top of one of the pneumatic car lifts; toggle the nearby panel to lower it.
You'll hear a radio broadcast from Nico, saying he's dug in at the radio station. After seeing one of his bombs at work, you'll soon come up on the alien assault on the radio station itself. For this skirmish, you'll have an extra command option available, for Nico to set of his explosive packs. If there are three or more aliens gathered near a pack, that's a good time to use it; otherwise you'll want to save them for now. Check your ammo before approaching the door, as another wave of aliens will airdrop in, including a Tech Commander, who will use the cutscene to build a turret. When you take out the rest of his squad, a Muton will engage you. There's a supply drop soon, so drown it in any grenades you have, and try to lure it near any available explosive packs.
Once inside the station, you'll find Nico has been infected with the Sleepwalker pathogen. He's been able to fight it, but he's almost succumbed, so he wants to do one last kamikaze strike to open the way to the Spire for you: you have the option to either let him do so, or command him to bail out at the last moment so he can help XCOM study the disease; if you do so, he will appear in a later Base Investigation-- otherwise, he'll be replaced by Private Jenn Eric. Afterwards, resupply and make any other changes at the aforementioned supply drop. Going forward, you'll see energy pods burst from the ground in the cityscape. Past a drop, there's a dead end to the left where an infected woman has committed suicide, and on your way back, a lone Sectoid will spawn from where the Shield Disruptor Pack schematic is.
The next section is a gauntlet of Sectoids and multiple waves of mixed Zudjari through several back yards; high-capacity mid-ranged weapons may be better here. At the end, your reward is your first plasma device in an open weapons locker: the Blaster Launcher! You may want to swap out a weapon and hold on to this, using any remaining Energy Packs to get it to full ammo.
Next, you'll be on a high path above a gas station: start firing on the aliens to distract them while Nico drives his car bomb into the station (you can get a little extra XP if you can kill some aliens before Nico blows them up).
Coming around a bus to the front of the supermarket, you'll see some Outsiders knocking some Sectoids around: a grenade-bait formation that would be good to capitalize on. Both on foot and by drop pods, more aliens will appear, finishing off with a Sectopod, which you definitely should use to test out your new launcher. Don't forget that back attacks to the rear Plasma conduits, especially with the Launcher, are devastating to Sectopods. There's a large Energy Pack and another launcher on the alien structure on the right if you need it.
Through the Supermarket and out the back, you'll meet up with Dr Wier, and go through a few more buildings, and to another Supply Drop. In a nearby corner is another Medical Pack schematic, if you somehow missed it in the nuclear missile mission, or skipped that operation altogether.
You'll pass Agents Kinney and Redmon's bodies, and go down a set of stairs to a Mosaic terminal, which Dr Wier will interface with. From the door above and behind, a number of Sectoids and Silacoids will spawn: defeat them all to continue. Follow the path and use the teleportation pad. On the other end, there is an Armor Piercer schematic nearby. Continue along the path until the windowed hallway where a guard platoon is visible on the other side. Enter the room and prepare to engage the alien guards, which include Elites, a Tech Commander (try to kill him before he can get the turret up), and a Muton. There are a few explosive barrels around the path that you can use to ambush it.
When that's done, continue forward to when you reach the Venn Gate, where Wier has gotten Mosaic to cough up a Phase Plotter transponder for you. When you grab it, the alarms will sound, and Axis will address you. Continue forward, and prepare for a few aliens who will engage you on some stairs. Around the bend, some more will fight from a balcony with another teleport pad. Take it through, and you'll be on the Spire's top level, where you'll find a last supply drop. Here, consider changing equipment and agents for either all out attack (two Recon snipers and either Tactician or Gunner packs) or pure defense (Supports or Commandos with shields, and the Guardian Pack). Your best bet is the Blaster Launcher, and either the Laser Pistol or the Laser SMG.
Now, enter the last room and fight the Titan.