The trick with Zen Daijun really isnt a trick at all, heres how it works;
First u have had to have done the mission
Second u need to do the +15 point attribute quest u get from...whatever its name is...the dragon horse like thing that was with u when u rang the bell in the Zen Daijun mission (he's at Seitung Harbor, over by his bell, behind the armor, weapons and dye traders)
Then once u can do Hard Mode with the character(at level 20) go back to Zen Daijun explorable area, entering it by talking to the gate guard at Seitung Harbor....then go in....kill the first normal level 3 animal u see....and its over...u have just vanquished the area.
The reason i dont really call this a trick is because during the +15 attribute quest with horse dragon dude u cleared the afflicted and the plague from the area....and besides the normal animals...thats all that was there LOL
NOTE: ONLY a FACTIONS campain character can do this, because characters from other campains cannot get that quest since they have their own +15 attribute point quests and must take the ones that are from their original campaign.
ALSO, a group trying to use this 'trick' must be comprised of characters from the Factions campaign only-it will not work otherwise.