Instructions
Grab an Assault Rifle or Submachine Gun. Rapid-fire weapons such as these will help when it comes to fighting the last boss. He spawns multiple copies of himself and comes at you from several angles at once. Weapons with a high magazine capacity and rapid fire will allow you to take down several of his copies without reloading. This saves you valuable time.
Stock up on Medkits. You can only carry three and chances are high that you will need all three. Try and save all of your Medkits for the final encounter. Note that once you meet up with Sgt. Stokes in Still Island, you will not encounter any more enemies until the final boss.
Use your Reflex sparingly. Hopefully you found at least a few of the Reflex Boosters. These increase the length of your Reflex bar, allowing you to stay in Slow-Motion longer. Using your Reflex strategically is essential to surviving this fight. Your enemy will attack in waves, with a couple seconds rest in between attacks. Use this time to regain some of your Reflex.
Keep moving. While there is a bit of cover to be had, your opponent moves quickly and can easily surround you if you let him. Counter this by continually moving and sprinting when you can. Again, do not use all your Sprint in a single burst. Try and save some for when you really need it.
Look for the consoles. You will need to activate three of them in order to reactivate the Telesthetic Amplifier. Your enemy will try to stop you at two of them. You will need to tap a button (varies by platform) to stop him and at the end, to point the gun back at him and pull the trigger.
Keep at it! It is a good bet that you will be killed at least once trying to finish this encounter. This is perfectly normal. It is the final boss, after all. If something doesn't work, try something else. Remember to reload during lulls in the fight and use rapid-fire weapons. Guns like the Pulse Rifle and Hammerhead can take out the copies faster, but require a reload much sooner. They also require a greater degree of accuracy from you. If you're a good shot, try using the Hammerhead. If you're not that good, use the Assault Rifle.