Motion Tracker - Exfil Point

Motion tracker device simulation - with a twist. Along with motion of aliens, this motion tracker detects projectile trajectories, hits on target, transmitters from the colonists, and sudden changes in temperature. Luckily, it has button linked to trigger on your pulse rifle, so you can use it to aim and shoot. Also, there is a fire selector switch to choose between single, auto, flame, or grenade fire modes. You can tilt your device to turn and aim, or choose turret mode to access on screen turn controls.
Your assignment is to protect exfil (exfiltration) point for civilian retreat. Visibility is zero, so you will have to use device assisted aiming and shooting. Your goal is to defer all approaching alien hostiles and spare friendly targets. In return, every colonist you save will give you personal ammunition supply.
How long can you hold your ground?