Soon as you open the door, don't go in. Position your squad on the rails of the ramp going up to your right. Then, Strafe to the left, take your Sniper Rifle out, and start working on the Atlus from afar until it's down. Your squad should keep the enemies below at bay, allowing you more than enough time to finish it off.
Now, move up the ramp, past your squad, and take out a few of the Guardians and Centurions below. Wait for your reinforcements to show upthis'll trigger more Guardians, Centurions, and Engineers to spawn from the door at the end of the ramp pathway. Use Liara's Singularity on the Guardians to leave them vulnerable, and Garrus' Overload on the generator as soon as they pop out, the discharge should take a lot out of their shields and/or health. Try to finish off the Engineers in a hurry, prevent them from setting up any turrets. Once you clear the ramp of any opposition above, gradually move yourself and your squad up to that position.
Once that's done, set yourself up there and take potshots at the enemies coming in from below. Try to take out the Engineers first so they won't set up any turrets, and be careful not to stick your head out for too long, in case you're within the sights of any turrets that're already in place. Troopers and Centurions will often try to come up the ramp, and stairs at the end of it to flank you, but you should be able to see them coming, as long as you're attentive.
Next part of the courtyard isn't so different. You can keep your squad below to draw some fire away from you as you quickly make your way up to the top of the stairs. Guardians will spawn nearby after a few enemies below are taken out, get rid of them fast. For the rest, just make sure you get the Engineers out of the way quickly, and don't let the Centurions get too close. It may help to assign Overload and Singularity to the d-pad to quickly stun them, take out their shields, or pull them out of cover
