You must enable the developer console (~) in options in order to use these commands. Bring up the console by pressing ~ which is located to the left of 1 on your keyboard. Load dust2 by entering:

map de_dust2

Once loaded enter the following commands in console:

bot_quota 30
bot_knives_only 1
bot_difficulty 0

Once the round begins have the terrorist bots stay in spawn by using the radio command x + 4 (or w/e you have it bound to) for "Hold this position." Plant the bomb and wait. Kite the bots around and BOOM!

This works best on classic casual, to set your offline game to classic casual in console enter:

game_mode 0
game_type 0


EXTRA: This knives only command works great for getting the Rampage and Conversationalist achievements as well. ^.^