The Freeze Ray is a 1 hit kill on the Energy Leech even on Insane difficulty
Well, basically a 1 hit kill. What you'll need to do is swim up the the boss and pull the trigger. You will get frozen, your ego will drop to nothing and your screen will be flashing red but you will see the the Energy Leech's health will completely deplete while you are frozen. Once you unfreeze, all you need to do is switch to a RPG and shoot him once and that will put his health in the red flashing state so you can start the QTE event to finish him off.
Just something I stumbled upon last night completely by accident and I reloaded to verify it works everytime and wasnt just a glitch. Hopefully this is helpful for those getting really frustrated at this boss fight. If you dont have the Freeze Ray, you can get one in the previous chapter.