It's been said in other topics, but Yatagarasu is actually one of the best eidolons as she comes with an ATK down debuff on her ability. If you don't main fire, definitely focus on her completely. If you DO plan on being a fire-main, then it's more complicated... Assault SSRs are definitely good, but again, Yatagarasu herself is good as well. What i'd do in your shoes is try to get as many copies of Yata as you can, but if you manage to get enough to buy her 5th and final copy before the event ends... don't. There's always a chance that you could get a copy of her to actually drop, thus saving you tons of time. The shop does NOT close after the event ends, and indeed that's how you get many many elixers. You can change 2/d for weeks, depending in how many materials you have saved up - I'm still turning in ones from the last 2 advents. Also that way, if you don't get a lucky drop, you can still get the final copy of Yatagarasu then.
TL;DR: For any element, not fire, Yatagarasu has a spot on your endgame team if you can MBL her. SSR weapon is only worth MBL if you are fire.
The weird situation I'm at currently is I have a lot of fire weapons with decent fire kamihime (Motu, brynhilde, amon, and rares), but all my fire weapons are 2-300 damage weaker than the SSR weapons I currently have (varying elements). My fire grid is 8 skill (5 atk varying ranks 3 hp varying ranks) at 750hp and 10484 atk, or highest damage grid being at 853hp and 11807 atk (I have 6 SSR weapons, only 1 being fire from this event).
Assuming this is Yatagarasu, as I'm not sure why you didn't just post in there, the eidolon is more important. The attack down debuff is more valuable than 1 SSR weapon, even if you us fire. 1 SSR weapon is unlikely to be the deciding factor to whether you win or lose as long as your weapons in general are leveled, while Yatagarasu's debuff can be. Regardless, you'll get another chance to max them again in the future when we get a rematch event.
If you re new then you definitively want to go for the eidolon. Even if it was a really good weapon as you evolve your account you will end up with a lot of different souls that might not be able to use the weapon, vs the eidolon which is way more versatile since you can use her in any team because of her effect.