@Capt_Gigglepants:
The former god was the "god of mischief".
He made the people fight each other believing they follow god's order / will / plan for the world.
So before fighting Arlong there will be situations where Cain needs to prevent wars, monster rampages and so on that are caused by Arlong manipulating the monsters and people while hiding himself.
As such before fighting Arlong little Cain will have to find him first while making sure that there won't be a huge war between several countries.
At least that is what I expect.
A former "god of mischief" just coming out and saying "Here I am. Fight me!" would be too dumb for a being who manipulated all human countries to fight each other while he just watched.