The lights are on
I just recently completed the game so it's still fresh in my mind. There are three places you need to hit the end boss. The best way to do that is from behind the arch you came into the map. I've tried to fight from behind the pillars in the middle of the field but died...
Useful weaponry: * Longbow grenades (The only ones you can hit it well with) * Sniper rifles (I think shock did pretty well) * 11001010 cannon or what ever the name was exactly (infinite ammo = very handy towards the end) * Rocket launcher
You can see a pattern here: distance distance distance.
Alright. The first thing you have to target are the glowing purple orbs in the tentacles. As long as the tentacles are around they shoot missiles at you in periodic intervals. And let me tell you those hurt! Only weapon suitable for this is the sniper.
Second thing is the tongue. The boss has a vertical mouth. Once you've destroyed the tentacles this thing starts opening on occasion and a tongue shoots out trying to hit you. You have to hit that. Best way to do it is the sniper gun. If you manage to time it right you can use the 11010101 or RPG.
When you have destroyed the tongue the eye part begins. Occasionally the eye opens and shoots a bright beam at you. When it does that the eye becomes an easy target. You can hit the eye in between attacks too but it requires rather good aim. From time to time the tentacles will grow back again and you'll have to shoot them off a few more times so it's important that you save enough sniper bullets for that and use the slower ranged weapons for the eye.