After the game messed with me with multiple banners with a single green orb, and a couple with none at all, I got Estive Elincia. However, it messed with me again with a +HP/-Spd spread. It's a superboon, but Spd is her best stat >.>
I'm tired of these bad natures. I don't think I have any optimal Rank5 unit at all, actually. My best ones are +Atk/-HP, when -Def or -Res would have been ideal. Even my potentially stronger unit, Winter Chrom with 3 merges I have on reserve, is +Res/-Spd, so a mixed tank.
The nature for your Elincia has some merit, in that it lets her score a tad higher in arena than if she had any other boon… but as a ranged dancer, she will make your score plummet regardless. Though they can be used to great effect in arena assault, where having multiple different singers/dancers can be benificial. Singers/Dancers have little reason to be involved in combat in the first place, so a bad nature for them isn't as bad compared to most others. But I get that it is annoying, especially on a unit you like.
Unless you mean 5 star exclusives, it might be easier to choose a unit from the 3~4 star pool and upgrade that one. You are more likely to summon multiples of them (due to their higher appearancw rate), and thus a greater chance at a perfect nature (and merges). As for perfect natures, them might differ depending on how you look at it. Sheena and Gwendolyn both have less than stellar speed (25 and 24 respectively), but that is their only stat with a superboon; which lets them score higher in arena. As such a suboptimal nature can be considered optimal. A bane in HP can make it easier to safely reach thresholds for skills like "Desperation", and can be useful for a receiver in "Infantry pulse" teams.
Anyhow, on the first round gave me 1 red stone and 4 green; 1 of them contained a "Sonya: Vengeful mage" (+res, -hp), the rest was generic. Sonya's nature is usable for ploying enemies, and instantly charging her special with the help to "Infantry pulse"… if only that skill was easier to obtain.
Got a "Micaiah: Summer's dawn" (+def, -hp) on the next round. Not the best nature, could be worse. Gave her "Wings of mercy", as it is a great skill for dancers in general. Should there be a weakened opponent next to the ally she is warping to, she can aslo try to safely pick them of thanks to "Hardy bearing" that is built into her weapon.
Amazing luck, but I'm not really planning to see for how long it holds up, so I will leave the banner for what it is.