I told ya: throw s**t at your players first and then help them wriggle out of it. If they make an effort. If not, watch them die and rot.
Remember this: what's challenging for a normal party is auto-success for marines. So they must be placed into dire straits where only the most heroic maneuvers will get them out of. Conversely, if a marine asks for something that should be near impossible for a human, they should have a chance of making it (if they don't threaten to abuse it).
Alex