Beating steel ball openings
The below openings are assuming your opponent started with Quick Supply Specialist and Steel Ball/Crawler as their starting units. This lets them buy both an extra crawler and a mustang on the first turn to make a quick steel ball push.
All of the below openings placements win on round 1, preventing the steel balls from leveling and snowballing out of control.
Once you survive round 1, round 2 can be used to shore up any weaknesses now that you know what side they are playing on.