Play 'Blacks Cats' on hardened and when you get to the part where you have to rescue the survivors, do what your supposed to do but you must kill all enemy PT boats no matter what otherwise you will die and have to quit and start over.

You should start shooting PT boats right after you see them. You have to hurry and shoot them before they kill you.

If you die, quit the level and start over again.

This may take a few attempts. I found this out on my own and did this and got the achievement.