She can move behind the rocks as everyone moves out. The elves are not gonna stay back once the fighting starts. If she were any further back she'd be standing with he back to a 30 foot cliff. Or she'd be down it where she can't do anything.I actually think Escella is already too close to the action. She should have been in that back line of folks. All of her stuff is ranged.
Edit to add: The rocky overhang (cave-like but doesn't go back far) is dimly lit the only reason it's hard to see into (aside from your spell covering the opening) is because it is off the road far enough to be concealed by layers of brush. Through the brush you could spot the orcs near the entrance but not further in.