I really enjoy the "Stranded" map, however once the team reaches the bunker it is smooth sailing and it is very difficult to die. Players quit from boredom.

What is the best configuration to make the game as difficult as possible in MP coop?

Limitting the amount of lives each person gets is a good place to start, maybe even just one or two per person. Maybe limit the respawn rates on health and other pickups until a certain score limit is reached (say every 1000 points?). Friendly Fire as well as falling damage adds to the difficulty too.

Just think of anything that you feel might make the game more challenging and interesting for you and go from there. :)