Why are you using Arthur if you want a sustain? Obviously with no healer, if you want sustain, you use Cass/Andromeda... and no, don't unlock Andromeda until you have the other basics: Cass, Mordred, Dartagnan, Gawain and Joan.

Anyway, I don't think you should be using Arthur in the first place. Since you don't have any debuffs in that group (aside from Satan who is off-element), Gawain + Sniper Shot (Dartagnan) gives you -40% Def and would increase your damage output drastically. That way, you might not need sustain.

You have a good amount of options, though. Like others have said, you can replace Sniper Shot (-20%/-20%) with Cybele (-15%/-15%, though for you it's currently -10%/-10%) which opens up the ex slot for your Soul. You still should probably run Gawain over Arthur, maybe with Joan's -20% Atk debuff... ... or just run straight up Joan with Gawain's Ambush.

Overall though, you still have so much you can gain. You have very little in terms of Eidolons or weapons (have you been leveling weapon skills on your Wind weapons?). That's something only time and dedication can fix.


On a side note, there's fun things you can do when maining Wind element. You don't have the pieces right now, but stuff like SSR Cu Chulainn + SR Freya's Intercept + R Puck's Berserk can give you so much damage over two turns it isn't even funny. Well, it does require good luck with enemies actually hitting Cu Chulainn... but even then, she'd go to town. Guaranteed triple attacks while doing double damage? And countering quadruple damage? Fun times.
Probably not worth running a R hime for that, but hey. I said fun, not the most broken thing ever.