How hard is it to give Eirika the infamous +12 to all stats?

I don't think it's possible to give her +12 in ALL stats unless you put her on a Tactic's team, and even then you'd need Legendary Lyn to give her the +6 speed buff from Speed Tactic (unless you foddered her off for said skill).

Titania gives Res + Atk + Def Tactic to Eirika, L!Lyn, and another non-infantry unit while L!Lyn gives Spd Tactic to Eirika, Titania, and the other non-infantry unit, then Eirika receives an extra +6 when near the other non-infantry unit, right?

I don't think the execution is that difficult if you have the necessary tools.

Use titania with the poleaxe refine and two different tactics in the c slot and sacred seal. Then just get a unit with the fourth tactic (unless it isn't that desired) and it becomes a lot easier. One of the units will get +6 to all stats when within those two units, and if Erika got those buffs too (which isn't too hard, 2 spaces is a lot of room), then having her within 2 spaces will get her +12 to all stats.

Bonus points if the unit with the +6 to all stats use a blade tome, or if it's corrin with the refined yato (and ally support, duh). Then Erika will get +20 to all stats when adjacent to corrin. Jesus she's on steroids.

I haven't used Eirika much so this is just the way it works in my head, but I imagine the easiest way is to build Eirika into an enemy phase unit who always moves with her team. This might involve taking advantage of her superboon in defense. Give her DC, QR, or maybe Vantage if you're assuming she's always got +12 speed. And I think you could technically do it with the tactics only on two units - two on one teammate, two on another, just make sure they're both within two spaces of Eirika *and* each other at all times. There's also the possibility of Delthea/Linde with Dark Aura, who could buff her for three out of four stats by herself when they're adjacent, although she'd also have to buff an additional nearby teammate to give Eirika the +12 bonus.

Edit: I forgot about Titania's refine, her effect is probably better for this than Dark Aura.

This is actually quite easy as long as you keep a T or a Square formation.

My team is :
Eirika : Sieglinde/Swap/Noontime
DC/Quick Ripost/Drive Speed/Distant Def

L!Lyn : Swift Mulagir/Draw Back/Moonbow
Sacae's Laws/Desperation/Speed Tatctics/Att Smoke

B!Ninian : Blarblade/Danse/Moonbow
Darting Blow/Chill Atk/Atk Tactics/Guidance

Archer Hinoka : Warrior Princess/Reposition/Moonbow
Atk/Spe Bond/Flier formation/Res Tactics/Def Tactics

This work really good. To be honest Eirika and Ninian are doing almost everything, I am planing to find better options cause L!Lyn is not that great but she is the only one having Speed tactics, but I won't fodder her. And I don't have many options for a flying grey unit. But I am looking forward a future staff flying unit