There is no summon for god's fish. The easiest way to beat him is to stack evasion to 100% and then you just need enough health to survive a roar. You have to avoid his body slam attack because that thing does 100% of your health in damage so it will always kill you if it connects but you can't rely on evade skill because he has an unavoidable attack that will dispel it and if he decides to follow up with a slam, you're dead.
For gear use leaf's ring, phantom cloak or abhorrent cloak if you think you can rely on regen, Sky knight's cap, target buckler, ring of sin (located past god fish, you can use dodo speed to run past him and then come back to fight him) and ring of emptiness to get 100% natural evasion. jub jub covenant can also give you evasion but that may not be an option if you're taking on god fish.