1. Make a ring of bunkers, leaving exactly 1 space between each bunker.
2. Put exactly 1 Marauder into each bunker. No more, no less, and it MUST be a marauder.
3. Rally each bunker to the bunker next to it.
4. When the enemy comes near any side of the bunker ring, select all bunkers and mash D as fast as you can. The marauders will cycle through the bunkers and fire immediately.
Why does this work? Because Marauders are fat enough to fill the space between bunkers, and because they have no attack animation time - they fire instantly. This is why they will fire as fast as you can mash D.