As the other solutions and cheevo description states do NOT get incapacitated.
If you nearly die jump into the body of someone else. If you fail you can simply restart checkpoint.
Now this level can be a giant pain in the butthole!!!
Here are a few tips to help you that really helped me do this level.
1. Jones is your best friend. (Astral projection dude)
2. Black is your 2nd best friend (Sniper lady)
3. Cole can be your buddy if absolutely needed (Grenade lady)
4. Everyone else is worthless
Switch to Jones and the very first enemy you can run up to and shotgun (LT) his head off.
Now as the next 2 enemies come round the corner use Jones's astral projection RB to highlight one big dude and then LB to shoot into his body, Now you will be given 2 options on screen X & B. Hold B to perform a blood ward which will basically freeze and enemies close. (THIS TECHNIQUE YOU WILL USE THROUGHOUT THE MISSION)
There does not seem to be any cooldown on this powerup so spam it as much as you can.
The next stage you will go through a gate, As soon as your through the gate press down on the D-pad to halt your team on the walkway.
If they go into the pool they will get slaughtered.
Now use jones method above to freeze enemies and use blacks LT to launch an explosive round at the dudes that explode.
Keep using this method until all enemies are dead and use cole only if you absolutely need too.
This is the hardest part of the level in my opinion and the 1st checkpoint aint till after this battle.
move to the other side of the pool and up the stairs and a few more enemies will spawn.
Eventually you will encounter a big behemoth. Same as previous levels destroy the 5 points around it's face and switch to Church (sword lady) and run up to it and press A to start the QTE sequence.
After this is done move on to the cutscene and then advance forward.
Stay as black and try to stay as far back as possible for the next battle which is much easier. Use ghost bullets (RB) to take out the flying dudes and pick off any other enemies.
Once they are dead move on and enter the tunnels for the end of level and cheevo pop.
This may or may not be the best method but it was the easiest method for me personally.
Good luck peeps.
**Why the negative votes??? If you want to down vote me then please grow a pair of balls and comment as to why it deserves a negative vote, give me some constructive feedback so I can improve it, or try writing your own solution if you think this one is worthless, if yours is better i'll quite happily delete this one.**