Once you enter the level kill everyone in there. I would kill the immortal first. The executor and bone golem can only be killed with "blood drunk" so aim for them. Once they're dead aim for the captain. The captain can only be killed with a heavy wrath attack (R + square).
Once you've killed them hydarnes will appear. Once this happens equip your spear. Begin stricking him with a heavy attack (note: only hit once then begin blocking his attacks). Now hydarnes is tough and faster than you so it can get annoying and he will summon immortals to aid him but only kill one or he will keep summoning them. Only the spear can break the armor. Once you've broken the armor you then may use any weapon of choice. He will then run to the cages and jump on them. Once this happens hide behind the pillars with bodies on them. He will then throw his fist in to it. Once that occurs run to the next one. When that fist is in the pillar throw your spear at him. Keep doing this untill he dies.