I did it at lvl 26 or so. I would suggest getting a fireteam that knows to keep phogoth alive and makes the first part a where you unlock the door piece of cake (I personally thought that was harder than killing Xyor). Also an arc shotgun and something with solar damage will help make short work of the knights and wizards (shotguns in general help). Though I highly suggest the arc shotgun due to the Knights arc shield. What class are you? And what platform?