If you have Prophecies you can level heroes to about 10 or so easily by taking them in the Nolani Academy Mission. Flag them to make them stay on the wall by Rurik and then open the gates and kill the Charr. You can easily solo the Charr with just about any character. Once they are dead, just map back to the outpost, rinse, and repeat.
Just after I bought Nightfall, I did the mad rush to get all my exsisting chars (5) their heros. Still lvl'ing them up using xp scrolls and killing bugs in the jungle.
The fastest way I found to lvl them up is to get to the mainland in Nightfall and make the rounds of the Marga Coast and Arkjok Ward using xp scrolls and the bounty to get Sunspear promotion points
Acolytes Sousuke and Jin weren't part of the "upgrade" to lvl 15 because, technically speaking, they're not required (not part of any primary quests or missions, and their recruitment quests aren't required either).