Wait, stop. This is not the right path, and I don't think you understand exactly how getting Souls is supposed to work (yet again, probably because nobody has explained it). First of all, you get Soul Points from fully clearing events and leveling up, but it's only 100 soul points per raid/union event and only 30 soul points per level, while the amount of rank points required to level increases. Therefore, your main source of soul points are ADVENTS, where you can farm up to 600 soul points per advent. You need to get all of them because you need to get all the souls. Why do you need all souls? You primarily want their extra skills and their mastery bonuses, to a lesser extent.
The reason we suggest to unlock souls in a certain order to obtain powerful extra skills that you will need for a variety of situations:
- Shingen's Provisional Forest - Increases the burst damage all all allies by 200%
- Mordred's Black Propaganda - Increases Max Charge Turn
- D'Artagnan's Sniper Shot - B-frame 20% ATK & DEF Down
- Vivian's Maiden's Prayer - Cleanses 1 debuff for all allies
- Gawain's Ambush - A-frame 20% DEF Down
- Cassiopeia's Chaos Magic - Removes 1 buff from an enemy
These are just some of the extra skills you can consider taking. Mordred is a good soul not just for her early-game debuffs, but because you can take Black Propaganda with you in the future no matter what element it is. Ryu-Oh does have some overlap, yes, but she is also famous for missing a lot, which can be fatal for a new player.
D'Art is particularly notable because she comes with Sniper Shot. Most elements don't have 20% B-frame debuffs. Again, it can be taken with you wherever you go.
And finally, SHINGEN. She has the best end-game EX skill, Provisional Forest, but the problem is that you CAN'T GRIND HER, you can only wait. The only (reasonable) source of Holy Soul Points are Union Events, and Union Events can give you a maximum of 200 HSP per event. The Shingen line therefore takes SIX Union Events to fully unlock. So all you can do is collect 100% of rewards per Union Event and wait (farm lilims if you are weak, do the low level experts/ultimate demons at the start of reset, etc. just get all the individual rewards). The only other source of HSP are from tower medals; except in edge case scenarios, you should never do this, because cost-to-return ratio is terrible, especially since it is eating into your next Holy Steel Argentum.
But let's say we just take into account your specific water team. You still shouldn't get Arthur, because where are you going to get the rest of your 50% defense down as a new player? You already have 35% DEF down in Ryu-Oh and Snow Raphael, taking up A+B-Frame. So where do you get the rest? The only frames left are {Water Resistance Down, C-Frame, Weapon/Summon Frame}. Excluding you getting an eidolon which can do it, this is why
Hercules is good right now, because you can buy her Soul Weapon which, when she bursts, applies between 15-25% DEF down in the Weapon/Summon Frame. This is by far the
biggest reason why people keep using Hercules, so get Hercules.
Finally,
never get Axe Caledfwlch. It's terrible. Every Soul has two weapons, one which gives 30% Elemental Attack (the same thing most eidolons give you) and one which gives 30% Elemental HP at max skill level. Axe Caledfwlch is Arthur's HP weapon. It upgrades Avalon's Guidance to give 20% D-Frame ATK, but as an attack buff, this is character attack, the same kind your weapon skills give you, which means it tends to be worse than elemental attack pound-for-pound. So in terms of damage, it gives less character attack less often than the elemental attack you can get for running... anything else that has that.
And before anyone argues that you have less character attack
now because you have Rudra and no weapon, it doesn't matter because Regalia is not cheap. For new players, it's especially hard to farm because they can't get MVP and have small AP pools for opening them. You will get a grid with a decent amount of character attack easily, so there is no point investing expensive Regalia now into a crap HP weapon. Two copies of Hercule's Axe will serve you much better.