What about a melee mode where all the bots fight against only the players?
so you could have maybe 2-3 players against 13-14 bots.
should be easy, all you'de need to do is make it so the bots don't attack each other.
or better yet: zombie mode, where all the npc's only use melee and they all are against the players, AND...they know where you are at all times and come in a never ending rush!
just a thought.