Stay within the doorway as long as

Stay within the doorway as long as you can so the door doesn't shut behind
you and lock you in. Make full use of this protection as long as you can.
Go for the head shot on the shotgun guy first, then take out the visible
stragglers. From the doorway, you should see two pairs of legs on the
upper level, one left side, one right. Take these guys out a shot at a
time.

When the area is clear (ish - there are two shotgun guys outside your
viewable area) edge forward bit by bit and more waves of guys will come.
Stay within the doorway and take out the enemies from a distance - feel
free to use the gas canisters and crates to assist the kills (don't waste
them though). You will get used to the order that they come in - because
you will do this more than once on hard difficulty!
When the 2-3 shield guys come out - this is your cue to bail. Run forward
and pick up any grenades and ammo nearby. Spin while you do this to take
out the two shotgun guys (if they stayed put). Now keep moving and throw a
grenade at the shield guys (keep firing on them so they don't block the
grenade).

Now you are heading toward the back left / right spawning area. In these
rooms the enemies spawn above and drop down to join the fight. There
should only be a pair of normal and a shotgun guys coming at this stage.

Stay at this end, under the walkway to guard from the coming RPG's. There
are two guys left above on the gantry. Travel between the two spawning
rooms and take them out while keeping your cover.

If you thing they are dead / dying WAIT! in the spawning room to see if
the machine gun nests open.
If they don't - go and finish off your last guy.

If the machine gunners do turn up out, throw a grenade at the nest, and
HIDE again.

DO NOT LET THE RPG SEE YOU!

Stay covered against the wall that would be closest to the front of the
room. As the grenade detonates, pop back out and through 3 more grenades,
HIDE again and you should hear the constant detonation of the machine gun
nest being destroyed!
WAIT, through one more grenade for luck (as the
machine gunner sometimes survives or the far one will run towards you!)
you do not want to risk being slowed down with the RPG behind you.

Now is your chance to run, from the room, through the machine gun nest and
out of the level