The most useful monster.

Djinn are good a bit later in the game, when there are enough of them to summon a large number of illusions. Then, the hero only needs to take Djinn and the Original for the illusion with them into battles :). Four slots, one Djinn each, for the "songs of the world," and in the fifth slot, put all the rest to copy illusions from the Original. And in the seventh slot, the Hero himself :)
I hadn't thought of such a tactic; I'll have to try it when I get a computer. In general, Djinn and Water Elementals are the main opponents of vampires; they kill them very quickly.