Exist several aways to do this, but a quickly one is:
Load up a Single player, Custom game, with the following settings:
Map type: Inland
Map size: Tiny
Reveal map: All Visible
All the other settings don't really matter. Then once the match starts, press ENTER again and type "steroids", this will make the enimies build units faster.
After, press ENTER, type " bigdaddy " and press ENTER again to spawn a race car.
This race car has a powerfull gun that you can use for destroy your enimies faster, usually just one shot!