Have you tried pulling back to the second ramp? If you fill six bunkers with marines and back them up with turrets up there, that'll keep off anything you get against you if you have two or three auto-repairing SCVs (watch out for muta splash, as that will kill repairing SCVs after a while. Having one medic hanging back can be an idea if it turns into a problem). Wait until it's four or so minutes left, and move out with a MASSIVE ball of marines and medics. Medics are important, I ran with a 1:3 medic/marine ratio. Constantly pumping SCVs is also pretty damn important - without doing that, getting enough units will probably be impossible.
The way I did it, I went for the leftmost hatches first, and then the top left ones, there's two hatches on each spot. Whenever you run into a spine crawler, you will loose marines, so you'll want to be pumping reinforcements non-stop.
Also remember to save, and reload if something went terribly wrong (a bunker went down/you overextended and lost half your marines etc.). Reloading doesn't fuck up the achievements, as far as I know.