There should be an npc summon in one of the cells before the boss gate. He helps a lot so make sure you are human. When you get in, stay on the platform and kill the first sentinel. With you and the npc, he should be no problem. Then hop down and play defense until one or more sentinel aggros onto your summon. Then just kill them one at a time. If you can get one to fight you alone, your summon can deal a lot of damage to the one on him while you finish off yours.