You finally really want to conclude your WoT series with the last book: A Memory of Light. You still have 8 Forsaken around, with whom the Light has to deal with in some way. You already got a good idea how Rand will balefire Semirhage by using the True Power. Since Graendal is linked to Rand's plotline in Arad Doman, it makes sense to you that Rand will deal with her there as well. You invent a nice plotline where luck, Nynaeve's capability and ta'veren will lead Rand to Graendal's hideout. Then you come up with a brilliant scene, where your hero brutally manages to outsmart a smarter foe by using force (but also clever mechanics of balefire and Compulsion). This scene will give you a chance to put your hero in an ethical dilemma, but at the same time get rid of Graendal, who didn't deliver anything to the plot anymore. You are content then, because you only have 6 Forsaken to still deal with.