|Est's new weapon allows her to punch holes in armors, as well as anyone else unfortunate enough to be caught in the weapon's effect.
Death Blow 3 or Death Blow 4 are the best options for this build. Assuming she has the weapon activation requirement met, she essentially has a 16 Mt Brave weapon; when combined with either of these skills, you get +12 or +16 total extra damage (depending on the skill used).
A number of B slot skills can be used here, but one good option is Hit and Run. With this, she can safely dive into combat, and return to safety after. Alternatively, she can choose a breaker skill, such as Swordbreaker 3. With many of the game's sword units having 40+ speed, having this skill can help keep Est safe from heavy damage.
The C slot is flexible, but flier buffs are highly recommended since she'll need to be around at least 2 to activate her weapon's effect. Goad Fliers is a cheap, easy choice that's found on her eldest sister, Palla.
Heavy Blade 3 is a powerful choice for a seal as it allows her to activate 2-turn specials such as Moonbow on that same turn, which would likely obliterate the opponent before they could retaliate. Attack +3 is also a viable option; a flat boost to Attack never hurts.