Act 1: Ashes - 3. COGs through the Debris
After the scene, run over the area behind the big house-like things. Flank that Troika and the guy behind it; I could kill the guy and then cut the gunner in half. Then keep going until three Drones appear, and just take cover and pick them off one by one. Move up to the fountain area.
Go in only when you have a full four Frags. As soon as you get in, jump back out, and run to the left (facing the way you came). There will be four holes in all, but you can change the order of the first three. From the left you will need to close two holes that pop up in the streets that you came from. Move back from the closest one so that you don't overthrow it. Then another will pop up where you are standing (oh yeah, make sure you come back to the same spot if the farther-away one opens second). Once the third is plugged, go to the other side of the street (left of the third hole) and plug up the last one before it opens, you may need to run.
Collect Frags and ammo, and then COG #6 in the corner opposite the fourth hole.
Go through the open path and choose the right path. When you reach the hall with the Troika, get out a Frag and get as far up as you can, and then let it go (aim as far as possible). Kill whoever survives (save two Frags) then go man the gun and wait for two Drones to rush. Then go out the door and go to the other side of the room. Stick close to the wall as you make your way into the little room, then run into the other one. Edge out of the doorway and take out the spotter in the broken hole in the wall up above. Then Roadie Run over to the big cylinder thing, and then hide behind the support to the right of the Troika. Get out a Frag and arch it behind the gunner to kill them both. Then waste no time running back into the room with the piano. The E. Hole will open where the piano was, so lob in a Frag. Then come back and fend off what remains of the rush. One last Drones runs from the door behind the Troika. Collect ammo and go through.