The Leviathan boss at the end of the game is without a doubt the worst boss ever created. There is absolutly no skill involved in fighting the boss, it is nothing more than blind luck. It has the ability to kill you in one shot by hitting you, an attack which is impossible to actually dodge or avoid, all you can do is hope that it doesn't kill you.
The theory behind hurting the boss is the shoot the glowing red orbs, however again it's nothing but luck as they're almost impossible to hit, even if your cursor is dead on them. They must have a ridiculously small hit box. There's no checkpoints at all so if the boss kills you in one shot, (which he almost certainly will) it's back to the begininning to fight him all over again.
Everyone who worked on this terrible boss should be deeply ashamed of themselves, it doesn't show the tinyest evidence of reasonsed thought, planning, intelligence or good game design.
There really needed to be some regular chimera shooting at you or something as you fight it, like the abomination battle. In my opinion the abomination battle was harder, more thought out and just plain better.