|There's absolutely no doubt that Est is a fantastic Brave Lance+ wielder. However, much like her fellow lance fliers, Est can run a melee generalist set quite well, especially if you already have similar units filling role of a Brave Lance user.
Notably, what sets Est apart from said fliers is (1) her high Resistance stat (enabling her to apply Ploys and tank red magic if you need her to) as well as (2) default access to a fair lance weapon in Slaying Spear+. Additionally, not only is this set relatively easy and inexpensive to achieve on a budget, but it also has the distinction of working well in higher-scoring Arena Assault environments due to the frequent appearance of high-scoring armors in the mode.
The frequent use of formidable Bold Fighter 3 or Vengeful Fighter 3 armors in defense teams may prompt you to have a few dedicated anti-armor units. Slaying Spear+ does just that, with Est being able to acquire it inexpensively by evolving and refining her default Heavy Spear+. With it and the correct passive skills, Est is able to initiate on and OHKO a variety of even the most highly-merged, Ward Armor-boosted armored units in tier 20, including Winter Tharja, Winter Robin, and Zelgius (and pretty much any Red armor for that matter). Additionally, the weapon itself sits at a pretty hefty 14 Mt, which will certainly be painful in combination with Est's high Attack. Though Slaying Lance+ is tempting as it is commonly suggested on most melee lance builds, Slaying Spear is already quite useful for reasons previously specified. Est's use of it is also debatable as faster competitors, especially Catria, can make better use of the lowered cooldown by doubling more often.
Reposition and Draw Back are pretty standard movement assists which can help your teammates escape enemy range. The choice between Glimmer and Iceberg depends on needs. The former has a very short cooldown of 2 and works well in conjunction with Est's high Attack stat Slaying Spear's damage scaling against armors, while the latter provides a nice catch-all damage bomb coming off of Est's also-nice Resistance.
Death Blow 3 is the optimal skill to improve Est's one-shotting potential. Many armors have kits that allow them to dish out powerful Specials on their first counter, and it's imperative that Est can kill as many of them with one hit as possible. The +atk boon helps further here, as does the refine. Her speed can be improved, but her B slot can be used to guarantee kills on swords that she can double without dying to.
Est's B slot passive skill is mostly free, though for cheap practability, Swordbreaker 3 allows Est to overcome her lower Speed and win matches against extremely fast sword units universally. This is especially important as units such as Ayra and Mia with Wrath could otherwise win regardless of weapon triangle advantage due to their powerful specials. Additionally, ORKOing sword units completely prevents opposing teammates with Wings of Mercy 3 or Reposition from ruining a favorable map position. More luxurious passives such as Chill Def 3 may be considered, and have great effect at further improving Est's ability to one-shot opponents, although this requires that she targets the enemies with highest defense first.
The C-slot is flexible, but it wouldn't be an Est build without mentioning Def Ploy 3. It helps Est establish further KOes on defense-fortified armors and support her teammates in getting their own kills. It can be used as the Sacred Seal, but Attack +3 would be preferred in that slot to increase Est's damage output in conjunction with the Ploy. If Chill Def 3 is chosen for her B slot, the ploy might lose some of its effectiveness, though.