I think its the final boss (has 3 arms and you gotta hit tthem when their moth is open).

You have to shoot the green glowing things on the arms to make them open. then throw the glaive into the electricity and then throw it in the middle of the claws.































Then when the boss opens his shield you have to hit him a couple times with the glaive before he kills you instantly.































Repeat three times untill he dies and you beat the game.