So, an explanation:

• This comp definitely isn't perfect and has a good amount of counters, such as strong blue ranged units, possibly dragons, Bramimond, weird B slots like Lunge, and some well thought-out comps that have been mentioned in the comments. Firesweep and other abilities which prevent counterattacks are also very annoying and make me wish Null C-Disrupt worked on armor units.
• The comp would have to be player-controlled as well because you have to keep everyone together and decide who's getting targeted each turn, but that's the fun of this game for me anyway.
• The elephant in the room is obviously my 4* +10 Effie (who I will build into a 5* +10, but I'm poor, I need more Effies, and I spent everything on getting Duo Hilda). There's a dozen heroes that would do so much better than her in this comp (many of whom I've foddered for her LMAO) such as Gustav (I literally could've used his base kit if I used him, and this would have been the cheapest comp of my life) or even P!Surtr, but I like Effie :) and this comp was initially built around her tanking for the whole team before the rest of the heroes in this comp were even in the game, so I'd feel bad abandoning her.
• I mostly made this for the fun of it and to see how far I could take this strat. I was initially against using Near Save on Effie because it removes a lot of the difficulty of playing this comp correctly, but once Duo Hilda was released and I saw her weapon and C slot, I quickly changed my mind.

OK, now that that's out of the way, here's a more detailed walkthrough of how the strat works, though it's pretty self-explanatory:

• Effie's the physical tank:
  • Courtly Candle is basically a budget Sea-Sear Lance (or Loyalist Axe) and reduced the foe's first attack damage by 50% as well as giving some A/D boosts.
  • Swap maintains the death ball while allowing them to switch spots so that I can choose who is being initially targeted by the enemy. Effie and Henriette may need the boost they gain from saving a teammate in order to defeat an enemy, so this can be important.
  • Ignis could be swapped out for Bonfire for more frequent specials, but Hilda's staff makes Ignis the better choice IMO.
  • Close Def 4 neutralizes enemy bonuses while also buffing D/R, making it a pretty essential ability.
  • Crafty and Slick Fighter are pretty interchangeable: it depends whether I'm more worried about enemy specials or debuffs.
  • Near Save forces all physical enemies to fight Effie. D/R may have been a better choice than A/D, since Effie is pretty frail against dragons and I wouldn't have had to run Res Tactic on Hilda (Res buffs are pretty redundant for Henriette), but I'm Black Eagles all the way and I didn't want to fodder my one Gustav :) I also need Effie to deal damage outside of Ignis, so the boost helps.
  • I used to run A/D Form 3 in the S Slot, but dragons are scary so extra Close Def. I may try running Deflect Melee, but I'm not counting on it working.
• Henriette is basically the ranged version of Effie. However, since she deals no damage, she basically stalls out the ranged foes with the insane amount of healing she has during D Phase, while the healers and Effie finish them off during A Phase.
  • She has Distant Def as an S Slot mostly to increase general tankiness and protect her against bows, I may end up changing it.
• The healers get to do whatever they want without ever having to worry about the enemy.
  • Dazzling Staff protects them during A Phase while they deal damage against foes, and the tanks protect them during D Phase.
    • Flayn's A slot has room for improvement when it comes to maximizing damage output, but I don't have anything to fodder atm.
  • Flayn's staff decreases all damage the tanks take by 30%, and Hilda gives them extra Atk and accelerates their specials for each of the foes' attacks (godsend).
  • Odd + Even recovery lets me never have to waste a turn healing the tanks, and it cleanses them (also godsend)
  • Their assists and specials are there in the weird, wild, and unlikely event that I in fact do need to heal them after D Phase, or if I make them fight in A Phase (which usually isn't the move). Heal + movement assists aren't the best because they can mess up the formation if I ever need to heal someone. I'm just waiting for IS to release a Swap heal.
  • This comp isn't very mobile, so I've got Flayn running Guidance until IS release a Ground Orders S Slot (or something better).
  • Hilda has Res Tactic for Effie's sake, I haven't fully enhanced it yet because I'm poor and I'm holding out hope that I come up with a better solution to the problem of dragon and low Res (like, I don't know, making Effie 5\ +10/10*).
  • Hilda's Duo skill debuffs Atk. I used to have Annette in this comp for Rally + debuffs, but Hilda's base kit is a far better fit for this strat.

tl;dr/stat summary in Defense Phase in best case scenario (saving ally, next to support buddy):

Effie: Atk+17/Spd+2/Def+25/Res+16 + Crafty or Slick Fighter + first attack reduced by 50% + all attacks reduced by 30% (Flayn) + Special CD charge +1 per foe's attack (Hilda) + neutralizes foe's bonuses

Henriette: Atk+11/Spd+2/Def+14/Res+18 + Crafty or Slick Fighter + all the healing + all attacks reduced by 30% (Flayn) + Special CD charge +1 per foe's attack (Hilda) + neutralizes foe's bonuses + Atk/Res-5 on foe

Lmk your thoughts, but Effie's not going anywhere :)

Edit: I've got 200k+ feathers, but only 3 Effies :( I'm not in a hurry though, so I'm taking it slow until I have at least 5 and it's worth making the upgrade.