Eliminate Remaining Xenos | Mission 9: Hope in Hadley's A: CM Guide

You'll get this objective as soon as you disconnect the third fuel line. I suggest staying close to your teammates; this will reduce the risk of receiving critical damage in the final part of the mission. Aim at Spitters and Lurkers first. Use your best weapons; you won't need the ammo later. Keep fighting until the mission ends, which is soon after the dropship gets back.