Da heck you are talking about?
"Hercules is great, but as long as your KHs already have def down fully covered, it's better to use Shingen." That's totally BS.
Who said you should only get Herc-relic if you don't reach def cap... ?
First of all Shingen will be too fast for his Team anyway, why should he get Shingen?
Furthermore with that team he can just run Hercules+PF.
In addition Hercules has a strong Nuke, your water team lacks dmg anyway, and is a strong tank.
Hercules is also enough to carry your team till AQ4 with relic.
And please don't MTix Aphrodite... That would be a waste. Why?
Snow Raphi and Poseidon are enough to keep you alive. Get Herc relic for more damage and level your water Grid.
MTix Asherah becaue Hercules is also a bit too fast for your water team.
If you don't want to MTix then wait till you get Asherah, Shiva or Neptune to replace Poseidon until she gets her AW.
I have the exactly same water team but Poseidon is in my sub and Asherah in main team.
If you lack the damage to brute force your way, then use Andromeda till your grid is better.
I would say you will need at least 38k attack, Herc relic SL20 and 9k HP+ to do everything without healer, disaster rags not inculuded.