For those of you who are struggling in the courtyard after you meet up with the friendly squad, here's a handy hint to skip the entire section!
From the checkpoint (should be as you come up the stairs to the friendly holdout), immediately run and jump straight out of the main hole in the wall. When you land, toss a flashbang into the enemy spawn straight ahead of you, and start moving around the left of the barrier - You may just catch the edge of the flash, but you'll stun 7 or 8 guys in the process.
Here's the plan... LEG IT. Run straight through the dudes as fast as you can and head to the left, taking out one or two late spawners. Head around and behind the shack to your left, and hide in the dead end. Give it a few seconds and you'll see 'Checkpoint Reached'... Nearly all of the enemies will disappear, your squad will move up towards the LZ, and you can get on with the level!
It may take a few attempts to pull off, but it's less frustrating than doing the same bit over and over again with no luck at all.