How do you beat inphyy's final boss on ninety nine nights (xbox 360)?

i read a forum sumwhere saying you had to get all s rankings on all of inpyys missions to unlock some special weapon or something but ive got s rankings on all mission except the last jungle mission i try to kill everything on the map but i keep getting interupted by the goblin king guy and then teleported to him and then i miss half the enemies.