CC is probably going to be fastest in terms of series exp./minute if that’s all you’re going for.

Daily Frontline is not bad either, since they added the daily series exp. bonus a while back.

There’s hardly a need for a grind at this point, though, considering you have at least nine more months to get it done. It’s less monotonous to go for some other PVP achievements/secondary goals and you should be able to get all the series exp. you need along the way.