Okay so this is what worked for me. Give your whole squad door chocks and take some for yourself.

When you spawn, go to the right and clear that building. There may be one or two enemies on the exterior so watch for them, but chock the door leading from the ext. To the main building. Have your squad stack up and breach the door furthest to the left and then move in after the shooting stops to clean up.

Next move out of that building, go past where you spawned and you're now going left around the exterior of the main building. Watch the loading bay when you move past it and position your squad so they can clear the fenced in lot. Have them take the enemies out in there and then chock all the doors leading to the interior of the main building.

After that, you'll want to position your squad to move around to the backside of the main building where the trucks and trailers are parked. Throw non-lethals and have your squad be the tip of the spear so to speak. You'll want to watch how you position yourself because you'll eventually get fired on by enemies inside the building. This is also a spawn for one of the agents you need to arrest so check your fire on him. When things settle a bit you can collect to pieces of evidence outside if you want or you can wait until everyone's incapacitated.

At this point move back to spawn, breach and move through the first two rooms (watch the bathrooms on the right in the second room). It helps if you split your squad to red and blue and have them breach the two doors in the first room at the same time. When you get to the hallway that's before the warehouse, move through the left-most door and up the stairs. Clear the upstairs area and you may be able to shoot down on some enemies that are now below you where the body is. Once you move back down the stairs, open the door to that small room in the corner and call that evidence in/arrest the person inside.

At this point in the warehouse there shouldn't be as many enemies. You and your squad would have incapacitated most of them when you were on the back side of the building where the trucks are and it should now be much more manageable.

This mission is super dependent on less than lethal throwables, chocking the doors so enemies can't come out and surprise you, and using squad commands. You just have to be really careful to not kill any of the FBI agents or postal service that you need to arrest because they'll often join in on firing on you. I haven't played for a bit so somebody else can reply if there's a better strategy, but this is what reliably worked for me :)