iruka is a free to gain ninja (you get 10 fragments every 3000 points refinement in ninja refinements). Anyways, the point is that against World Boss you want ninjas with more than one standard attack per round (like suigetsu), ninjas that boost other ninjas (like guy) and ninjas that owns high combo chases (10, 20, 30 triggering ones, like iruka or tenten gnw chases).

what are your current ninjas? for a while, when i was low level, i used with azure fang haku, suigetsu, guy for example.