Ok to get stronger fusions for kurando and yuri simply go into there personal choose fusions and then level them up using sole points but in order to do this with yuri you must have the third form of a particular element to do it up so say you want to level up your fire fusion you must have the third fire fusion in order to do it I hope this where as kurando only has one fusion so you caan do his up straight away I hope this helped with them strong bosses where you need a better fusion