Move on up and get into the office area. Loot the safe and check the area. You can also look out the shutters to get a view of the battle you're about to enter. When you're ready to go, move on out to the right and hack the terminal to boost your sniper rifle damage.

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A bunch of Blue Suns mercenaries are waiting for you. The first waves are near the first set of crates and there should be a few mechs. Just take cover and blast right through them. Watch the right side. A lot of mercenaries are probably still dug in on the upper platforms. Climb up and over to scan the right side and take them out. There's a PDA you can hack here too.
The next area is the final fight. Harkin will call up two heavy mechs to fight you. They are also at a really close range. Just hang back and focus on taking them out one at a time. Use overlord and incinerate if you have them. You will also have to watch their machine gun fire. When it stops, you need to pop up and take a few shots.
Once you manage to take out the first heavy mech, the second will die as long as you can stay alive.
Harkin will put up a lot of raised platforms. I assume this was supposed to be a puzzle, but it's not. Just go to the left and climb the stairs that the crates make. Kill the two light mechs at the top to talk to Harkin.
Garrus will want to shoot out Harkin's kneecap at the end of the conversation. Paragons need to use the interrupt to stop him. Renegades can let it happen.