The Kobra of Death 

And here you have it.  A Kobra with 3-4 dust and 20 gas with only 5k no starting units.  Immediately use these units to pummel your enemy.  The 3-4 supporting dust are important- they should be used to help out the kobra in case it is attacked by early saws or dust from your enemy.  Note:  You can substitute a mino + 4 bikes or missle tank + 4 saws for a Kobra + 4 dust if you so choose, but remember Mino's are very slow.  It probably would work well only on knife fight using a Mino, but otherwise stick with making a Kobra.  Good luck!

4/4