Try an aoe spell? go to it without Sebille in your group(keep her far back so she doesn't ask to talk, and then fight everyone? There are so many ways to go about that quest I wouldn't be surprised if it bugs out.
Sebille already talked to the guardian. When we come up again, Saheila demands the tree to be killked. We go there, fight everyone defending it and still there's no option to damage the heart. Nothing does anything. AOEs included.
