Well, to remind people...
Arvedui's plan is ti beat the Prophecy of his name by creating a new Kingdom, so he will indeed be the Last King of Arthedain, and become the First King of the new Kingdom (as opposed to the other interpretation, which is that he is the Last King because everyone is about to die)
Having failed to unify with Gondor, he now plans to unify with Cardolan, which means convincing all the Lords to unify under him. Baronar is a powerful Lord, with faint Dunadan heritage, and was willing to listen to your cause, so you were going to convince him first, to be the first step of the way!
"They told the Tale well and with detail, all I can ask of you is that you help out the world of man one last time this age. I know we are scattered and divivded (sorry but I love that sentence in the movie) That we have called upon the aid of our elven allies too many times already. But as you well know human civilizations are like the storming sea's always in motion and hard to prodict. We don't have the luck, although I assume it can be bitter as well, to be blessed with immortality. This is all together the greatest strength and weakness of our culture, one weakness which you do not have. Humans are driven to become as famous and sometimes as powerful as they can become in their short life. This makes us mostly think about ourself and less about the greater good for ourself and for our species. But with Arvedui's plan we would start the reuniting of the human kin and it might be the start to a thing, we all dream about the unitation of the world of man.
But failing to aid Baronar now, when it needs aid the most, would surely make this plan fail before it has even get of the ground."
*pauzes for a moment*
"As I have heard you have lost many lives in the aiding of man and you have very lil men to spare, since you are under the threat of attack yourself. But maybe you can aid Baronar in a different way, what Baronar needs above all right now is the cure to that disease not elven troops. Ofcourse elven troops would be greatly appreciated but the cure will be a long time investment, into an even longer timed investment, The Possible Unificatio of Man"