Load game with the command line: hl.exe -dev -toconsole. Then, press ~ if the orange text on the screen does not disappear. During gameplay, press ~ to display the text prompt and type in sv_cheats 1. Press ~, then activate the following cheats by entering the corresponding codes. Note: You can only use cheats when the maximum players is set to "2":
CHEAT FUNCTION - CHEAT CODE Toggle God mode - godmode <0 or 1> All weapons - giveall Toggle no clipping mode - cl_noclip <0 or 1> Toggle opponents ignore character - cl_notarget <0 or 1>
Reach the two first doors one on top of another that are seen when you come out the end of the tunnel . Climb up on each other until you reach the top door's floor (or hook up). Click it and enter to find a fun game where you must throw the grenade (there is grenade ammunition in there) into the hole on top and get it inside the glass box.
After killing enough aliens with the airstrike with only two people on the map at the time, the monsters will spawn out of the ring around the outer area and there will be text reading "Monster revenge".