I'm not sure that there -is- a 'fast' way to do it, but I can give you my method.
It essentially becomes a situation with a bunch of straight-classing. Have everyone change to the same class, and then have almost everyone sub Fundaments (Mettle for Ramza, of course). This would be, ideally, a whole party of Generics. One or two should sub Items instead of Fundaments to heal as necessary. JP Boost is helpful for speeding it up, but not necessary.
In battle, have Ramza kill all but one or two enemies, and simple weaken the survivors.
Have everyone run around using Focus constantly. This will build JP for everyone in the class you're using (I usually start with Knight, then Black Mage, since they have to be mastered to get the class). Keep a running tally of the JP you accumulate by checking your Status every now and then. If any of the enemies start beating up on you (Especially with you leveling Black Mage or whatnot) then just have the designated Healers toss potions. Since they don't have Focus, they should be using weak attacks on each other and healing that damage to acquire spillover for Ramza and everyone else.
When you have enough JP to master Knight or Black Mage, or enough to get the other required classes to Job Level 8, then finish off the other enemies. Just make sure the initial kills all go to whoever you want to acquire Dark Knight.
By the time you actually get the last job to Level 8 (Or Mastered), you should easily have over 20 kills.
Now, have fun. At this point, any of the generics you were using can now unlock Dark Knight as well (More than likely, anyways), just have them kill twenty enemies and you're good to go.
(|)(|) Free loot, give 'em the boot. (ò.ó) Sell at the store, go back for more.