Retreat From Fire (20)

Cause the Alien to retreat using a molotov

See the "A True Engineer" achievement for info on where to find the Molotov blueprint in mission six.

Once you have constructed a Molotov, throw it directly at or very close to the Xenomorph and it should run away.

You can also do this achievement in Survivor mode with Amanda as soon as you start the game. In Survivor mode you can automatically create a Molotov without the need for a blueprint as long as you have gathered the appropriate crafting materials.