If you're going to max out, you're better off making a solid core first, unless you already have one. It'd help to come to a decision based on having a set team composition to be able to rely on in Arena and PvE content consistently (e.g. a rgb core with a spare team slot).
Yeah it’s hard to answer without knowing my teams, I definitely get that. I do have a solid core of every type of team, multiples of them, in most cases. I’m T20 consistently and borderline top rewards for AA, so I’m not really needing to boost my arena scores. Just trying to identify what next to invest in.
Honestly Cherche should be able to fulfill Frederick's role as a strong axe user unless you really need a cavalry axe, and given the trenches becoming more common in battle, flier mobility gives a lot more leniency. Very solid direction to go in next.
Other than that, Soleil/Fae/Ares are probably one of the most consistent choices. In this regard, Ares can edge out slightly over the others, as he can give a lot of instant burst offense through his qp set.
Adult Tiki and Soliel are both excellent units. I recently built my Adult Tiki (+Def, -Spd) and she is a tank.
But since you say that you've already have their roles covered, Cordelia would be a great unit to upgrade, especially since she's been demoted to 4 and 3 star. Though it'll take some heavy investment in order to get her to her full potential.
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