The best tactic was to build asteroid engines and ram your asteroids into the enemy asteroids. 45,000 credits for the blueprints and 190,000 to build, but that's still cheaper than Mega missile (500,000 credits) and can't miss. As long as you have more asteroids than the enemy, you can win this way
Some of the aliens actually had ways to counter asteroid engines. The Ax-Zilanths can teleport their asteroids to get out of the way. And the Tylarans have asteroid engines that go faster than yours, up to speed 8.
Nuclear missiles were especially good too. Another secret feature: Asteros ore actually has a half-life. If you don't mine it, it decays on its own.