We do not have any particular hints as to where Moronihah was stationed, but it would be logical that he was at Manti and that his force was the one bypassed when Coriantumr entered the land of Zarahemla so quickly. Having an army from Manti at his rear was always the danger in Coriantumr’s tactic, and it appears that this time the danger was realized to terrible devastation of Coriantumr’s army. Moronihah is clearly cutting off the retreat up the valley, and to have been able to come down the valley it is most probable that he came from the head of the valley, which is the location of Manti.
Verse 31 tells us that the Lamanites could no longer retreat in any direction because they were surrounded on all sides. We know of the armies of Lehi and Morianton on two of those sides, what was on the other sides? Coriantumr had led his army down a river valley, and therefore there is elevation on the two sides, and certainly a river on one side that made retreat difficult. In addition, there were those other Nephites who were in the land but had not been able to be called into action who would nevertheless be able to slow down a disorganized retreat through the more difficult terrain over the mountains of the valleys. In this way, Coriantumr was surrounded by the land as well as by the armies of the Nephites.