You'll be fighting three sentinels in this room; all are fast, all are powerful and all of them have long and wide range. These are pretty bad news for melee characters. As soon as you enter the mist, Yahim will ambush you. Block it with your shield or roll away to avoid damage. Do not fall off the ledge; otherwise you'll have to take on the other two sentinels at the same time. Time your attacks and heal whenever he finishes his swings. If ever Yahim is thrown out of the ledge, watch him to see his position. He will be able to jump back to the ledge and attack you at the same time.
It wil be tough dealing with them alone. One good strategy is to summon mage NPC and tank the damage by using a shield. You should make sure that Yahim is dealt with in the upper ledge; the battle will become harder if you reach the ground and Yahim is still alive. Don't rush this battle; just block/evade then attack only when necessary. They don't have that much HP but their attacks make it a real problem.
If you're able to defeat Yahim, get the aggro of the remaining sentinels by attacking them yourself. As long as the enemies are focused on you, your summoned phantom should be able to hit them with their attacks. Remember to attack whenever you can as well; the faster one of them goes down, the better. This will also ensure that the aggro will remain on you.
If ever the phantom is vanquished before you can do some real damage, you'll have to rely on proper timing of evasion or blocks to survive. The spinning attack is probably the most dangerous as it leaves you with little space to evade and it hits multiple times. Another problem is that if all their attacks connect, your stamina will be depleted, leaving you vulnerable.
Another attack to watch out is when a sentinel leaps in the air and slams its halberd down. If hit, it deals tremendous damage and any following attack will surely kill you unless you've healed. Just remember to attack only after their attack land. Don't try to interrupt their attacks as it will be disadvantegous to you.
This battle is frustrating and you will die several times because of your miscalculations or the enemies are just plain unforgiving. Nevertheless, you can still do it without a phantom ally. It's doable but really, really frustrating, especially for other Classes.