This is very easy - in fact the only ones you cant while do while idle are the 1v1 maps
for the 1 v 1 maps:
set single AI to lowest difficulty and game mode to assassination
build 2x air factory a few mass and a few energy, then have ACU and fabs assist factories.
Build bombers only. when you have approx 20 bombers send them to enemy base to target enemy ACU - you should be able to win these in 4-8mins each
For all maps with 3 or more total players
set single AI to lowest difficulty, and opposing team.
Set other/all other AI to custom (highest possible skill, rush tactics), on your team. start game. go watch youtube, while your AI kills the enemy. these take 5-7 minutes each usually