Just use them as support. I use Micaiah and Inigo, then I swap out either Ares or Aversa depending on who would be more useful. Both my Eirs have their base kit while one has Res Tactic and the other Def Tactic. Works out well, especially when giving your other three units +10 HP and Res with any buffs what so ever.
Double Eir for AR - what build?
Next week is Light season with Eir as a bonus unit, so maybe I can manage getting into tier 20 if I use 2 eirs and give out my light blessings.
Problem is, I have no idea how to make a double Eir setup work. I obviously need to give both dead princesses reposition / draw back, but I feel like I should probably somehow modify the second's Eir kit to make it more effective, but how?
I don't have any inheritable seasonal daggers.... wait, I do have Kitty Paddle, Goodie Boot and Lethal Carrot, but I'm not really willing to fodder them :/
The rest of the team will probably be a light blessed dancer (/L.Azura if it's still water season), Aversa with axebreaker and W.Fae / Celica / Nailah / SavagePain Lucius / ...?B.Veronica?
You could switch swift sparrow for fury on one of them and use a ploy or two. (You could also keep swift sparrow and still probably ploy most units that you need to thanks to bonus stats.)
Exploit the heck out of the extra res from the blessings and enjoy the 20HP heal from double sparkling boost. Maybe bring your healing tower to stack it all on.
If you use galeforce, maybe give one of them a smite.
If you don't have a non-flier ranged unit, give one of them Iote's shield so that she can initiate on bows if needed.
I noticed that you named Nailah as a candidate. If you stack the res blessings and give her def and res tactics and noontime and atk smoke, she can potentially tank 3-4 attackers...you might want to get her an atk tactic and maybe even a spd tactic as well.
...or just have L.Azura dance her! :)
What would be some good galeforce users? I only have a B.Roy that pitybroke me and a few Cordelias I could fodder off, but to who? Caeda? A cavalry unit with a slaying weapon? Having enough atk to activate heavy blade but not enough to one-shot the opponent so the unit can hit twice and actually proc Galeforce seems a hard feat to pull off. Especially since you also need enough speed to double
So, if you ask me, there are a few prime candidates for galeforce in AR.
Cavaliers are really nice for mobility; their disadvantage is that they can't use flashing blade, so L.Eirika is really nice because her extra charges don't require having more atk than the opponent (which is problematic against someone like Lilina who has poor physical bulk and a ton of attack). If you don't use her, then you pretty much have to sweetspot the atk in that 55-59 range because any higher and squishy ranged units die in one hit. (Some will die in one hit even to 55-59, but that's as good as it gets; there's no getting around this problem entirely.) Also really good is L.Ephraim for forcing doubles regardless of spd. Otherwise you're looking at figures like Oscar, Peri, etc. who can use slaying weapons to avoid needing Ostia's Pulse or heavy blade + QP. B.Roy is convenient because heavy blade is on his weapon so he can use QP. You really do want a one-round galeforce.
If it's not going to be cavalry, I think B.Celica might be optimal. Double Lion is really good for almost throwing speed out the window (ex: if Ophelia+10 gets too fast); it also has very good potential to get through Myrrh and green wary fighters (including Surtr) in one round. She can switch to slaying edge for consistent 4CD galeforce, or if there's pulse support, you can do what I do, keep Royal Sword and use flashing blade. That creates two conditions for galeforce to activate: if she outspeeds (even if she's not fast enough to double, because flashing blade only requires 1 spd higher, and Double Lion means she still hits twice), galeforce activates, and if she can't be fast enough, smite someone to be near her when she attacks, and galeforce still activates because of Royal Sword. Thus I have sometimes honed her atk and spd specifically to have Aversa panic her so that she wouldn't OHKO, and I was still able to galeforce.
But if you don't have B.Celica, probably Lon'qu? Because he's insanely fast and his prf makes a 4CD galeforce and he doesn't hit overly hard if you pick the right A-skill.
There are some unorthodox strategies you can try using bold fighters, but you'll need armour march + smite on a teammate. And I suggest pivot on the galeforcer, in case you want to snipe an opponent and run away. Again, the point of this strategy is that there's no stat check; bold fighter grants extra charges so galeforce activates if they don't have guard. The two ideas I've come up with are Sheena and LA Eliwood. The idea with Sheena is to give her brazen def/res and to buff her atk if she needs it. Having atk in the 40s means that she can avoid OHKOs on most squishy units. She can maybe take out the units that threaten her most (maybe some Lilina? but axebreaker can be an issue if she doesn't have +spd, full merges, and maybe a buff. Aversa also carries axebreaker fairly commonly), spread atk smoke, and tank the rest. Maybe give her a savage blow to keep Veronica busy. Similar idea with LA Eliwood, except his atk is better so maybe he uses fortress def/res, and also his spd is better so he's more likely to double wary fighters (not that it really matters...the most common wary fighter is Surtr)