How to kill with a knife in Counter-Strike: Condition Zero?

I am now on the mission 'Havana' in the hard missions mode. One of its challenges asks me to kill an enemy with a knife. I have tried several times with a tactical shield, charging into an enemy and then opening the shield and then attacking with the knife but I am always shot down. I also tried cheats, where I make the enemies spawn knives only but somehow, they also are able to spawn pistols and I lose again. A few times, the cheat actually worked but the enemies are still faster with their knives than me and they kill me anyway. How do I complete this challenge and kill someone with a knife?