This is earned on Mission 1: Form The Vanguard.
The final fight of the mission will pitch you against two jamming ships and a light cruiser. You have an optional objective to defeat the jamming ships first, in order to call for reinforcements. For this achievement, you have to do the opposite - destroying the the cruiser before the two jamming ships are killed. Note that your squadmates will be trying to kill the jamming ships, so you don't have long.
On lower difficulties, you can just put max power to shields and attack the cruiser at point blank range. Target the weapons first so you're taking less damage, and do a quick retreat and return if your shields get very low. You have a repair droid and your squadmate will resupply your missiles, so you can really just blaze away with little caution.
Note that you will need to complete the optional objective for
so you will need to run the mission at least twice. It's up to you whether you go for this first time.