Cerberus has a vested interest in stopping your attempt to get krogan support for the turians on Palaven.
They’ve taken control of a massive, defunct ground-to-space cannon array on the krogan homeworld of Tuchanka and have found a way to make it operational again.
Cerberus is now using the cannons to clear a path for an inbound ship to bomb the krogan resistance.
Taking back the cannons and shooting down the Cerberus vessel would keep the resistance in the fight, while also throwing a wrench in Cerberus’s plans.
SECURING THE CONTROL ROOM
Shepard sets the tone for this op right away with a simple phrase: “Kill anything that gets in our way.”
The first marked objective is to eliminate any hostiles within the control room and give the massive super-guns something better to shoot at than friendly forces.
Cerberus does not start off fully aware of your presence, so use this as a chance to get a free kill or two to start the battle.
Once you attack or are spotted, Cerberus moves to repel you immediately. Expect a mix of Assault Troopers, Guardians, Centurions, and Combat Engineers.
Remember, you can take the side passages into the control room, or you can brave the exterior area and push directly through the bulk of their resistance.
However you manage it, once the control room is clear, Shepard suggests using the superguns on Cerberus vessel before it’s in bombing range.
Check near the targeting displays for the guns for an Old Grid Schematic, used for a side quest on the Citadel.
Take advantage of the lack of Cerberus interference at this time and explore the map for items and Medi-Gel. Once you attempt to use the guns, they will return in force.
Old Grid Schematic location
RESTORE POWER
Predictably, Cerberus is one step ahead, having sabotaged power conduits for the cannons. These conduits are located inside the blasted out bunker Cortez dropped you off by.
To get the cannons back up and firing, you need to undo whatever it is they’ve done, which means a trip across the base back to the new objective.
Cerberus throws Nemesis snipers into the mix at this point, dropping them off atop the piping over the central exterior. Don’t let them get comfortable up there.
Resistance near the objective can be fairly intense.
Once you’ve dealt with the forces near and inside the bunker, shoot out the locks on the power conduits and activate them.
RETAKE THE CONTROL ROOM
With power restored, you must once more dislodge Cerberus from the control room.
It’s actually a good idea to rush the squad over there as quickly as possible, as it is likely Combat Engineers will be attempting to set up turrets inside the control room.
Denying them this will make life much easier.
Once you’ve resecured the control room, activate the cannons and watch the fireworks.
You can see it displayed on the holographic HUD in front of the controls, or if you’re fast, you can get outside and watch the ensuing explosion in the sky.
Mission accomplished!