Rockslide megabattle
The 'rockslide megabattle' is a superb battle against up to 39 enemies in the rockslide area of level 2 ('Halo') where Marine survivors are holding out. Easy to set up and rich in tactical possiblities, it's hugely replayable and you could basically think of it as a hidden mini-game. The combat is far more intense than normal, and you'll have great fun fighting alongside Marines or going solo. The battle was actually my original reason for creating this site.
As well as advice on setting things up I offer some specific battle plans and tactics, though I suggest you leave those alone for a while and have the fun of exploring your own ideas. Fellow pistol fans should be sure to check out the Far area defence sometime though, for a very special experience.