How can i save my marines?

For the achievement where you need to keep all the marines alive on heroic, how do you do this? That’s all they are good at o_O.

Micro.
I kid. I kid. Some Starcraft player is going to come through here and think “Yeah that joke was the -Yoink-!”

http://www.youtube.com/watch?v=xdmEK3VpZew

Here you go.