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Boon+ Spd Bane - Def

Skill Set

Hero Build Overview/Explanation

IVs:
+Spd/-Def for more consistent damage against Spurn, Repel etc.
+Res/-Def to give Tatiana some truly terrific magical bulk (even against Guinivere)

Weapon: Blarrabbit (+Spd/+Res)
The rabbit tome may be an odd choice, given that its effects only apply during the first combats in both PP and EP. But in combination with her A passive, Tatiana gets +10 for both Atk and Res while healing off the damage after combat.
The stat refine should match with the IVs.

Assist: Ardent Sacrifice / Positional Assist
Tatiana's default assist works well enough, since she heals off for 10 points during odd turns. As alternative, Reposition or anything else can be used.

Special: Iceberg / Glacies
Tatiana comes with a very high Res, so Iceberg or Glacies are natural choices and allow for good damage.

A passive: Atk/Res Push 3
As mentioned, this skill pairs very well with the rabbit tome, and it's also a good budget option, requiring only a 4* Brady.

B passive: Renewal 3
Keeping Tatiana on max HP is essential to maximize the Atk/Res boosts from her tome and her A skill.

C passive: Atk Opening 3 / Spd/Res Oath 3 / Rouse Atk/Res 3
The default skill is Atk Opening 3, giving Tatiana some support capabilities. You could also fodder a Dithorba or Spring Saleh for the respective Oath or Rouse skill, which benefits Tatiana herself (if you don't mind to spend some grails and feathers).

Sacred Seal: Spd/Res 2 / Atk/Spd Solo 3 / Spd/Res Bond 3
Any of these seals improves her key stats and will work fine.

Other options:
Atk/Res Push 4, Atk/Spd Push 4 and Lull Spd/Res are premium options to maximize Tatiana's offense, coming at the cost of a worse HP recovery.
If you have a spare Eir, you could change Renewal for Mystic Boost, which pairs even better with the rabbit tome and her A slot. It also removes Tatiana's weakness against the many adaptive dragon stones.

If IS ever releases an Atk/Res Push seal, you could change her A skill to B Duel Infantry 4, for better scoring and more raw stats.