The location the Divination leads you to is different each time you zone into Riven Earth. The proximity indicator will fill to indicate you are on the right track, initially blue and turns red as you get even closer. Zone in and out of Riven Earth (from Rata Sum) until your proximity indicator shows progress. It is possible to get the indicator at about 40% when entering Riven Earth. Once it fills completely, the Facet of Existence will spawn. The boss should not be much of a challenge provided you don't accidentally aggro a nearby group, although its Protection Prayers skills can make it time-consuming to kill.