Also this tempest made me realize that I don't really have a solid answer for red mages besides making azura tank them since Ike gets doubled when celica initiates with full health unless I give him a speed seal and buff.
Don't you have like 5 Cordelias? A =Atk 5* Cordelia can ORKO Lunatic Celica at full HP. -Atk would need +3 attack and 4* Cordelia, Donnel, Est with a Brave Lance, and 5* Hinoka would need to a dancer to help out. 4* Cordelia's special in that you could have her sit on a defense tile with someone with Hone Attack 3 behind her or Spur Attack if you can work that, have Celica attack with Cordelia taking negligible damage because of T-Adept 2 and being on a defense tile, Celica receives Ragnarok's -5 HP recoil, and Cordelia can KO Celica since at full HP, Celica survives with 5 HP. Also, I swear you said you had a +Res Cordelia, who would only take 5x2 on a defense tile as a 4* compared to =Res taking 9x2. 5* +Res Cordelia takes 0x2 damage from Celica at full HP.
As a free unit, Donnel's the cheapest way for anyone to deal with Celica and most red mages. Next up would be Est if you give her a Brave Lance from Donnel or someone. After that, well, for melee units who can 2-hit KO her from what I remember, 5* Camus and Sharena and if you were able to summon them: Azura, bride Charlotte, Ephraim, Effie, Luke, Ogma, and summer F!Robin. All of them, but Azura, Effie, and summer F!Robin die to full HP Celica and I think some of them still die if she's not at full HP without a Sapphire Lance, T-Adept, R Tomebreaker to stop from getting doubled, a ton of resistance buffs, and/or standing on a defense tile. All of them will need dancers to give them another turn since Lunatic Celica having 38 speed means most people will not be able to double her let alone escape being doubled by her.
Lunatic Celica's stats pissed me off that I ended up making a rant about it detailing how by default, almost all of the blue mages die to her if they're not on a defense tile except for =Spd Linde and because of R Tomebreaker, Odin. With skill inheritance... Way better outlook for them even if they're not on a defense tile. Here's a post where I go over what the blue mages need to deal with Celica:
https://serenesforest.net/forums/in...rture-fttobin/&do=findComment&comment=4907614, which can also apply to the other red mages since if you can murder full HP Celica, then you can probably murder the other red mages.
TL;DR: T-Adept and R Tomebreaker destroys her and Odin's actually useful for once since you can actually slap T-Adept 3 on him, do your usual -blade buffs on him, and he can counter-kill Celica at full HP. Cheapest skill inheritance option since Odin just needs T-Adept 3 while the other mages need to inherit both T-Adept 3 and R Tomebreaker.
Edit: =Atk 4* Odin with T-Adept 3 and an Eirika or Ephraim buff, +3 attack, +4 some other stat, is only off by like a couple of points of damage from destroying every red mage if they were +10 merged which is probably close to the Lunatic trials enemy stat boosts of around +4 to all stats. The highest he leaves is Sanaki with 5 HP thanks in part to her very high resistance (and Odin's crappy attack). Everyone else who survives has like 2 to 3 HP. Just giving Eirika or Ephraim a Fortify Res 2 sacred seal is enough for him to wipe out all the red mages.
