Have the max amount of bots all set to terrorist (bots wont work as survivor).
Put them on professonal and set it so they use mele only.
In more settings set mp_infammo to 0, mp_damagefactor to 3 and mp_zombiedmg to 1.
the super armour in the game is to moderates the damage you take from the zombies
making it easier.
people who want a challenge dont wear it.
Also start the game with the map called zm_l4d2c1m1 and it will automaticly camge to the horde event itself.