Try having the monsters continue to target PCs after they go down. Every hit on a PC who's at 0 is an auto-fail.I feel like it is too easy for PCs to stay alive. I would prefer that when you hit 0 hit points, you have X rounds and then you're dead.
I've never studied statistics, but aren't the odds exactly 50-50?As it is, the odds are that you are going to stabilize without help.