As the description says, beat this level on Hard and don't become incapacitated. You're squadmates can be incapacitated... it's just you that can't be. If you think you're about to get knocked out, quickly press A and switch to someone else before it happens. Otherwise, you can restart from the last checkpoint.
This level requires you to defeat some larger beasts. The trick is to shoot them in the back, preferably with Church's grenade launcher. The other thing that may be helpful is that the beasts always go after you when you attack them, so you can station your squads (using the commands) to a point behind the beast, and lure him away so that it faces away from them. Your team can do some damage that way. It also allows you to get Church around behind it while you're luring it away, and then switch to her to unload with your grenade launcher.