I think the plan is to draw Elayne to defend her city. Which will draw Rand to defend it in her place. In other words, the plan is to draw both Rand and Rand's armies to Caemlyn. As long as Elayne is alive she won't give the city up for lost, and that has to be key to Demandred's plans (which is hinted at in LOC). He is a general; his only two roles in the series worth mentioning are his skill in war and his desire to kill Rand (or his desire for Rand to die, period).