Single Player
Skirmish
Team one: Inactive AI
Team two: Inactive AI
You: Spectator
Start the game
Once in the game press enter to open chat and type in /cheat
The game should respond with Cheating is enabled!
There is a new yellow icon to the top left. Click it to open the cheat menu and check the box Global spec control.
From here there are 2 things you can do. You can either enable Quick build in the cheat menu which removes the metal cost of everything and has everything build instantly and you can manually control both sides to build whatever you want.
The other path is to open the cheat menu and use Unit spawning. Choose a category (say Striders), then select a specific unit/building (say Detriment), then select which player gets the unit(s) (say Null AI - Inactive AI 1), move the slider to adjust the number of units (say Count: 10) and and click the red wrapped gift icon, then click on the map and 10 detriments will spawn for inactive ai 1.
At that point you can give either side whatever you want, select everything and use attack move in the center of the map and the two armies will meet and fight.