On the last section of the last mission, you must activate eight conduits in three minutes. It is difficult because every time you activate one, it releases a difficult monster. The following is an easy way to kill them and complete the mission: at the start of the last mission you will see weapons on the walls. Among them is the Carbonizer (shoots a continuous beam). This will kill them easily. Note: It has fifteen shots with every reload and it requires all of them to kill the aliens. Make sure you do not use it until this part of the mission.