This is where you truly shine.
Make sure that you start building off of the closest wall, cave wall, or other such, and keep going.
Make sure you're aware of CAS, and that it could come at you nearly any-time. As such, remember to make or destroy pylons as you go. (usually if there's a drone, they will take care of this, but they may ask you for help in terms of plasma.
Keep compartmentalizing. Once you see the totality of your work that you've compartmentalized the WHOLE FUCKING MAP into death rooms full of traps and choke-points, there is NO better feeling than that.
Once you feel that feeling, your work is done as the xenos will almost surely win, but keep doing it to make it even more impossible for the marines to get ANYWHERE without being noticed.
The task WILL be daunting at first. But your hard work WILL pay off once you see the marines struggle to make it past all the compartments.