Raam is the final boss. You are almost done with the game Meh. Best strategy is IGNORE EVERYTHING. Dont worry about your partner and keeping him alive, ignore the chopper, and all other distractions.
When you head out, you dont want to run around, just hide behind the barrier right in front of you. Its got lights so staying there will protect you from kryll and is a good place to hide when your health gets low.
I found this battle easiest if you have the torque bow. If you do, shoot him with it. When the torque bow arrows explode the Kryll fly away. Once they leave him just light his ass up with another weapon, preferably the lancer. If you dont have the torque bow, then use grenades as they have the same effect and make the kryll scatter. When the kryll are gone, i would aim for his head. Try and use active reload a lot because doing this successfully causes more powerful bullets.
If you dont have the torque bow or many grenades, this take a lot longer. You have to wait for the kryll to leave him and attack you. Stay in the light behind the barrier and they wont hurt you but it's a bitch waiting for him to walk through lights causing the kryll shield to leave him.
Obviously Raam makes his way towards you, so when he gets too close, jump the barrier you are hiding behind and run down the center of the train to the identical barrier at the other end. There are lights through out the center of the train so running in the center will ensure the Kryll wont attack you. Just repeat hiding behind the barriers at both ends and focus on attacking Raam.
Just be patient. It may take you a whole bunch of tries to do this. Best thing you can do is watch your health. Duck back behind the barricade a lot so you dont die. Just work on kicking his ass when the kryll arent around and eventually you will get him. Good luck.