Get the Green chest to replenish your health. You will approach another battle ahead.
Before getting into the middle area for some fight, open the treasure chest that contains a Phoenix Feather.
Now you have to fight Centaurs and about two dozens of Sentries. This might be a little hard if you don’t have any of your weapons upgraded. The sentries will surround you and the best way to get rid of them easily is through the Army of Spartan magic. Once they’re dead, focus on the Centaur. After the Centaur’s defeated, go up, open the chest and head to the next area by using the stairs.