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.