Unload a full clip into one of the blue bits, causing it to explode, reload and kill the attacking Helghast while the Arc Tower's panels close. Reload when possible so when the tower's covers open again you will have enough ammunition to finish the job properly or you will suffer the ignominy of seeing the damned thing open only to find you have a few rounds left. Watching the reload animation while the conduit mocks you with it's weak points certainly isn't fun.
Once the Arc Tower is ruined you will be attacked by an ATAC that will swirl in overhead and move with all the snapping behaviour of a fish, but a heavily armed and angry one. Calm down dear, the ATAC's initial attack only takes place in a cut-scene that heralds the end of the level. While watching the loading screen tilt the Six-Axis controls and see that the screen you are looking at is actually a 3D hologram. Now they really didn't have to do that.
MISSION THREE: VISARI SQUARE
Follow the waypoint either by dropping down through holes blasted in the floors or by taking the stairs should you want to come across more graceful. You'll have to engage Helghast across the street who stop their attack a lot quicker if you move between cover and push on up to their position. Follow the waypoint until you come to a room with a shattered ceiling and kill the Helghast waiting in ambush above, and then jump down the hole in the floor.
MALIK BACK ALLEYS
Once you fall through the hole you'll find yourself in a storage room where some Intel can be found. Leave the room following the waypoint and take care of standard Helghast troops as you do so. Follow the waypoint up a set of metal stairs and you will see a door blocked by electricity.
Blast the nearby blue power box and your path will be cleared. Go through the door and down some stairs and you will encounter Helghast hiding behind pillars. Grenades are a great way of getting them out of cover, or you can wait for head shots. Wipe them out (all of them) and move on through to the next section of the mission.
This very short section will see your waypoint suggesting that you take a ladder, but you can also drop down a hole in the floor where you'll find some crates that you can smash to reveal extra grenades. Either way you'll need to follow the waypoint and take a ladder so you can move on to the next bit.
UNION STORAGE YARD
At last you get to regroup with the Alpha boys and will have to fight together because you are about to face a mass of Helghast. Note that Alpha decide to take cover behind sandbags and you can to, but you also have the option to be a great deal more cunning.
There's a ladder near the sandbags that leads up to a level above Alpha's position. If you climb up there you can get he drop on Helghast who will attack after breaking through a wall. You can kill them in safety and when you start to hear a grinding sound look for a wall that is starting to spark and ready a grenade. When the enemy APC breaks through chuck the grenade and you will annihilate an entire squad of troopers.
If you took position above Alpha you can also run to the right and duck near gaps in the wall where Helghast will appear totally and be unaware of your presence. Alpha will eventually open the door to the left of their position, so run through it ignoring any other Helghast since you have your own mission to think of. Collect the Intel and move on through to a room with a ramp that leads out to a window.
You'll need to defend the square from counter-attack and can do so either by taking a ladder up the back of a small booth in the centre of the square where you'll find a Sniper Rifle, or by using either of the two Machine Gun emplacements, or by just taking cover where possible and running amok with standard weapons and grenades if you are playing on the easiest difficulty setting.