What is a good Nino + Eirika build that allows Eirika to also take advantage of her Sieglinde (+Eff)?

I thought the only way to give Eirika the good buffs (+6 visible buffs) is to put her on a Tactics/Mixed team? Have her and Nino be the only infantry units, then give Eirika Spd Tactic and Def Tactic, that way she can increase her attack, speed, and defense?

Dual tactics and DC on Eirika, run mixed team, I personally don’t like running Nino with Eirika, I run another enemy phase unit with Def/Res tactics on Eirika.

It's mainly on how you distribute the tactics buffs imo. I would give Nino Atk Tactics, so Eirika can receive the +6, and Nino can benefit from the +4 from Sieglinde, though this does mean that Nino will miss out on an extra 2 atk if you gave Eirika Atk tactics, but that would make her Sieglinde's +4 atk meaningless to the entire party. Eirika can run Spd Tactics, and that'll make Eirika insanely fast alongside Nino. After that, you can run the Def Tactics SS on Eirika, so Eirika can have a better time against other melee units she's supposed to save Nino from. This also allow Nino to have at least a little bit of bulk against archers, and DC spear users. As for Res Tactics, I don't know.

I use Eirika as enemy phase unit though , mix team.
+Atk Eirika
Rally Def/Res, Steady Breath, Quick Riposte, Tactic Spd, Seal Close Defense, Aether.
Nino has Rally Spd/Def, Tactic Atk.

For Nino : Buff blarblade to KO one enemy, let 3rd ally reposition her back.
For Eirika : let them dual rally, stay next to each other, Eirika on front to face the enemies, she gets total 60 atk/48 spd/46 def if enemy initiates, she charges special skill ,quick riposte to gain hp back and defeat the foes.

Edit: that stats from +10, can replace QR with Renewal/Wing of Mercy/Sword Breaker ,up to you.

1. I recommend Eirika + Tailtiu unless you're merging (and maybe even then, but Tailtiu is such a gold mine) because generally speaking your blade mage is the offensive powerhouse who blasts everything to bits and Eirika is there to cover in case she can't
2. If you go with that, then I would put Titania and another non-infantry on the team. Triple tactics? Very yes. The reason is because if you want Eirika to be an offensive powerhouse, she shouldn't be giving the tactics; she should be receiving them twice - once directly, and a second time through her weapon and a nearby ally who also received the buffs.
3. If you're not going Eirika + Tailtiu you can still do tactics as long as the remaining members of the team aren't infantry. F.Grima is a great choice because she can be an anti-B.Lyn and give everyone extra spd and HP during earth season (something which Eirika and your blade mage will both love). Flier status also helps get everyone around.