Did you play that Terran 3 mission in the original Starcraft where you have to survive against Zerg for 30 minutes?
The noob way of playing that mission is spamming missile turrets, bunkers packed full of marines, constantly repairing everything, and sitting put, waiting for evac in constant terror. Then the Zerg overrun you.
The correct way of playing that mission is assembling a strike squad of 6 marines and a couple firebats early on (like, at the third-fourth minute), dosing them with stims, and killing off the Zerg while they are few. Then go get some tea while the Zerg never attack you because they are dead. Because you killed them dead. And dead Zergs don't attack.
The morale of that mission and this post, lost on the majority of players who played it: the best defense is a good offense. And "good" means quick and lethal.
(you can also do that in protoss final mission in scbw, to a slightly larger scale. I don't know if wol/hots/lotv kept the tradition)