save up your coupons and wait for him to be redeemable as fragments in an event, dont go for the rng because you may spend lots of coupons and not get anything but if you wait for fragments then it is guaranteed, not much other best way to get him just have to wait until he is put back in events as fragments

Events are the events in hot topic ?

Yes. I pieced my Shisui slowly and painfully. Spun the Lucky Stars Wheel where he was a prize for a lucky spin and at the same time he was available as a redeemable ninjas in the same event. Next event he was at Lucky Board, got some fragments off him from that event too. I forgot the other events he was on. I finally pieced his last 10 frags from last Great Plates event and got 4 extra frags off him. He is a great ninja and makes your life a bit easier.