In the vision given to Isaiah, Yahweh speaks to the armies of Elam and Media as if they are there listening to him. (See: [[rc://en/ta/man/jit/figs-apostrophe]])
It is understood that they are to attack the Babylonians. Alternate translation: "Go up and attack the Babylonians, you soldiers of Elam; go and besiege the Babylonians, you soldiers of Media" (See: [[rc://en/ta/man/jit/figs-explicit]])
Here "her" represents all the people who are suffering because of the Babylonians. Yahweh will cause them to stop groaning when he sends the armies of Elam and Media to destroy the Babylonians. (See: [[rc://en/ta/man/jit/figs-metonymy]])