Light 'em Up (10)

Use the flamethrower

At the start of the tenth mission, you will receive the flamethrower as part of the story.

You can also earn this achievement in Survivor mode. To ready the flamethrower, hold LT, then squeeze RT to spray.

Use the flamethrower directly on the Xenomorph before it gets too close. 40-50 flamethrower fuel should be enough to cause it to retreat. When it runs away, you'll also get the secret achievement “Back Off”.