Quest "Sealed King" - How to access the Philuscha King?

[thehintsch]

I went to the Ashra-am Baird Outskirts area (I guess this is from the free DLC?) and did the world quest line there. Put the White Moon's Soul thing in the ruins with the drawbridge in the north-east and now the quest tells me to "defeat the King of Philuscha on the islet".

I see the pool of water in the central small island and that it has a tunnel which seems to lead to the boss. With the magical rope I can only swing down to the round stone plate in the middle of the pool but the tunnel is out of reach from there. How do I get to the boss?
It looks as if I should either increase the water level or raise the stone plate or something? Is there a switch somewhere that I missed? Please help :)