Seeing his illusion have some effect, Tanner will continue to concentrate on the image of the Black Dragon swooping down on the gnolls, and he'll focus the dragon's attack on the spellcasting cleric (if he can see him), hopefully forcing him to cast a spell at the dragon or abort another spell that he was trying to cast. And as he concentrates on the Dragon's swoops and dives, its gaping maw, glinting teeth and hissing black acid, Tanner will use whatever skills of Performance, Charisma and Bluff that he can bring to bear with his illusory Black Dragon.