>>309743451
you have to unlock stuff. to do it, you need to find those balls with miniatures of objects inside. after that, you can either buy a debug gun in one of last levels of game or find it in one of sandboxes, and then you can spawn dumb 100 nullmen vs 100 headcrabs scenarios anywhere you want.

i guess you can try downloading a 100% save if that's not your thing, but honestly, this one sandbox where all of the stuff is hidden in a neat way (redacted chamber?) is just so fun to beat.