Open the console and type in:

mp_autoteambalance 0
mp_limitteam 30

Then start a server with bots, and add in 25 expert bots, turn off all bot weapons so they use only knives. Set max players to 30, and set bots to all join team terrorist.

Now you've got massive swarms of "zombies" against you and maybe a couple friends. Good luck!

meh.. play gmod zombie servers instead. Though, it is annoyingly hard to find a decent server that doesn't have the 1 guy that's been playing for the last hour or so and is at the top of the points chain...

I have a life, so I'm not good enough to play on the internet. I mostly play LAN games with my wife and some friends who come over to my house. That way, 4 or 5 of us can play without having to do 2 on 2, we can do 4 on 30.

And we don't have to download anything.

If you get toed good for the bots, try giving them grenades too.

I never turned off grenades, but they don't seem to use them when they don't use guns. You think you're getting too good for them, and then you go a couple rounds where your whole team gets wiped out. And then you can't afford to get the machine gun, and you run out of ammo pretty quickly. Then you keep dying. It's fun to hose them down with the machine gun, and you feel like you have it under control... then it all turns around. Lots of fun.