I'd go for Niko. Tiana can hold her own when it comes to nuking. Plus, you have Lapis if ever Tiana falls short in damage. There is also that accessory that gives you the same passive as Sylvie. Damage taken = MP regen. IMO, this skill is better on Tiana since she has lots of HP compared to Sylvie.
you've already got Lapis and Tiana to damage groups of enemies. Niko also gets decent group magic, but she can also deal lots of damage with normal attack. her 1st finishers is very useful if your running low on MP, damage + bind.
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