|Eirika makes for an excellent supporting unit, capable of buffing her allies for multiple stats. With her refined Sieglinde, she can then take advantage of both the buffs she gives to them, and still benefit from any buffs that they give to her. Because of this, Eirika is very customizable, and work well on mixed teams. This is so she can either give or take advantage of Tactics skills going around on her team, or she could even get indirectly boosted from specialized team buffs. It should be noted that a lot of Eirika's skills are customizable as well for this same reason, but what matters most is that she has some buffing skills to give to her allies.
Weapon: Sieglinde is Eirika's weapon of choice. It buffs any ally adjacent to her for +4 Atk, letting her be a great team supporter. When given its special effect, she will gain an in combat buff to her stats equal to any field buffs on any ally within two spaces of her. With just Eirika's base kit alone, she can buff +4 Atk and Spd simply by standing next to an ally. With this effect, as long as that ally is near Eirika, Eirika herself also gets the +4 Atk and Spd as an in combat buff. So not only can she support her team, but she'll also be able to help herself and perform even better in battle, making her a stronger combatant as well.
Assist: Swap is an option for Eirika's Assist. This is essential if Def/Res Link 3 is used as Eirika's B skill, as she can switch places with an ally and buff herself while also buffing her ally, which then grants Eirika a combat buff as well. She could use it to help get into position to tank an enemy hit, as it's best used in tight spaces. However, should any other B skill be used, a dual Rally like Rally Def/Res is great to use too.
Special: Moonbow or Glimmer both work as Eirika's Specials. She'll want something with a fast charge that can give her the upper hand in battle. Moonbow lets her break through sturdier foes, while Glimmer helps secure the victory against matchups that should be favourable for her. Use whichever is preferred.
A Passive: Distant Counter works as Eirika's A skill to ensure that she'll be able to counterattack regardless of range. She'll be able to retaliate against ranged units this way. Given that she'll be using her Assist on the player phase, she'll want an enemy phase A skill, and Distant Counter is a top notch choice. Atk/Spd Bond 3 is another option for Eirika's A skill. The nature of her weapon's effect requires her to be close to an ally, so this skill aims to boost her Atk and Spd to greater levels. With just her base kit and this skill, she can potentially reach +9 Atk and Spd in combat. +4 Atk and Spd buffing an ally, which Eirika then acquires for herself, and then since she's next to an ally, this skill grants another +5 Atk and Spd. The choice between Distant Counter and Atk/Spd Bond will depend on whether the player wants Eirika to be more well rounded and fear almost no ranged enemy, or improve her melee performance even more.
B Passive: Def/Res Link 3 is a fantastic option for Eirika's B skill. If Eirika uses a movement Assist, like Reposition, then both she and her ally gain +6 Def and Res. This works perfectly with Sieglinde's effect, as this one action will grant Eirika and her ally a great defensive field buff while also granting Eirika an additional defensive combat buff, effectively giving Eirika +12 Def and Res. However, this skill slot is flexible, open to be whatever else the player needs. Notably, Chill skills can help cement Eirika's role as a supporting unit, letting her debuff an opponent further away without even having to look at them, while a -Breaker of some kind can help if the player needs specific unit types to be taken down.
C Passive: Hone Spd 3 is a excellent option for Eirika's C skill. She comes with it naturally, and when paired with her weapon, she can buff +4 Atk and Spd to any adjacent ally, which she'll be able to gain for herself as well. However, this skill slot is flexible, but it should be for other buffs if necessary, like Tactic skills or Fortify skills.
Sacred Seal: Quick Riposte 3 is an option for Eirika's Seal slot to help improve her combat performance. This will allow her to perform a follow up attack when she's attacked. Eirika will likely be supporting for the most part, and will use her Assist to buff her allies on the player phase, meaning she'll likely see combat on the enemy phase instead. However, this skill slot is also flexible, open to be other Hones, Fortifies, or if the team allows it, Tactics skills.