Instead of some arbitrary "it ends in 20 minutes" time limit, maybe the big monster is indirectly making it's way to a certain location, in which you have to stop it? Of course, time could be extended by distracting it from time to time. Respawns are nice considering the length of the game, but perhaps with a lengthy respawn timer. Maybe like, 20-30 seconds. Or something that'll make people really not want to die.