There are posts on reddit about this, and a video. In short, Ethel is invisible, standing on the short wood bridge (behind and slightly to the right of Mayrina when you are standing in front of her)
Just use any AoE on the bridge (I used a flame arrow - when I selected it and aimed at the bridge, a red target circle showed up) and she appears. She is also bugged and doesn't fight back
even though she is red. You are not in combat but can just attack her or shove her off the bridge to save time. Unlike the regular, unbugged fight, when she gets below 10HP, she DOES NOT
beg for her life, so you can't get the charm (it doesn't seem to give an ability point anyway), so you might as well just shove her off the bridge.