Top down can be accomplished with grapling hooks. Added bonus is the Speacialist also has a shotgun.
From what I've learned from watching military docs and so on (not old enough to join) and some thinking, I've got this:
A) Stack up on the door:
Minimum of 2 men (never breach with a single person! It's suicide) one on either side. Automatic Rifles or Bursts are closest. One is designated to toss a grenade (whoever has the scope will be given this in a 2 man breach, multiman breach whoever is not right beside). Other man breaches first, or in multiple person situation one is designated to enter first. (These can actually be assigned beforehand if you are clearing out an entire building. Thus you don't have to be constantly working this out)
Having four people would be quite effective I think; two on either side of the door aiming at the sides of the room, one to chuck a grenade in, and one to go in.
B) Grenade, Grenade, Grenade:
Soldier designated to grenade tosses a grenade. If you know there are enemies, toss another one or even two depending on the size of the room. Two for a small room (About 3 meters by 5-6 meters) 3 for a medium room, and for a large one you should use as many as you feel necessary. If the room has windows and you can get someone in position to toss a grenade from outside try to do that as well. But be careful: A grenade tossed into a wrong window or wrong angle will result in it flying through the doorway into the hands of your teammates.
Remember: You do not have to stand in a doorway to toss a grenade. Stand off to the side with an angle such that you can toss it into the room, preferably off a wall. In a small room a grenade anywhere will kill or wound anyone in the room. You can also charge the throw with the right mouse to plop it a few feet from you, then quickly back into the wall.
If you are tossing several grenades don't take turns tossing, you may be killed by your buddies grenade. Preferable to toss at the same time, or if you can't, one tosses it far into the room, one tosses at an angle.
C) Breach:
The man who did not toss a grenade, or if he tossed the grenade first, enters and moves to a corner (whatever side he was on is the corner he goes to; if he was on the left of the doorway he goes to the left corner, if he was on the right he goes right. While he's moving fire fire fire. If you don't see a target just keep firing. If you see a target fire while you start pointing the gun at him. A large room is the only one that requires aiming. The one who last tossed moves in right behind the first breacher not firing until he has moved past the doorway. He moves into his corner and does the same as the first.
REMEMBER: As soon as the grenade(s) go off you rush in. You hear the woompf and your inside shooting.
D) Clear:
You have breached the room, but you aren't done. Move from the corners to cover and continue into the room. If something moves, shoot it. If it moves or makes a sound again, shoot it again. If it isn't moving shoot it. (Luckily in this game there are no civilians you might shoot, so every target is a bad guy in a building that's not manned by friendlies.)
E) Exit:
As you saw in the *cough* breaching video, you call out that it's clear, stack up at the door, call out that your coming out, and come out. Continue until you've taken the building.
If it's a large room, or large enough to sneak a grapple in, use it. If there's a breachable door and an open door you watch the open door, and breach and clear through the breachable door. If you can come from the top of the building, and preferably have two squads available, one squad breaches and clears from above, the other from the bottom. If you have armor of any kind they should lob a set number of shells in. Flashbangs are also extremely useful.
Basically: You want to disorient the enemy as much as possible, and you can do that just about any way you can think of. After that it's all about overwhelming firepower; put so many bullets into a set area that you're going to hit someone if they are there. Spray and Pray
Aiming is best if it's a long room, in a small room aiming just slows you down.