Indeed, Yata and Jorm have the best Event SSR Eidolon Effects
Ixion is a pain, I've cleared all the Ragnaroks I've challenged with my Fire team, but I'm not sure I can deal with her
You should for go first on Gawain for her Ambush, then Mordred and finally D'art for that Sniper Shot, what I said is most gacha SSR's at level 40 have the same stats as Mastema Lv85 and some even more (1200+ AT), but then my advice would be to first focus on the SSR Weapon, grab an Eidolon for the 100 Jewels and go for the SP since we're about to get yet another Legendary soul (Hercules) in the long run it won't be a waste
Just after, maybe if you have left over materials grab more of those weak Eidolons, remember that the Eidolon Trading System will be around soonerish and you'd want to have a few SSR event Eidolon left overs to trade for one with superior stats