Unit Suggestion: Reina, the War-crazy

You act like Reina, with the right weapon, is the only unit capable of handling a subgroup. Felicia with the Kitty Paddle can do the exact same thing, but far more effectively than this suggestion for Reina can, since Felicia's RES is notably higher. Though with Felicia, you can add Warding Blow or Warding Stance to increase her RES ecen further while doing it, assuring that she takes all but no damage from even the nukiest of mages, without leaving a major weakness unattended. Or for Cavalier units, the Ridersbane in the hands of Abel, or Clair, or - for the sake of argument - Camus could easily sweep through Cavalry teams. Even on Mathilda, the only unit who learns it naturally, it can easily break a Horse Emblem team, arguably the most notorious of team to fight. And Armored units are far from the only kind of unit that could survive anything Reina could throw at them. Most Cavaliers could, as well as a large number of even mid-tier Infantry. Chrom would laugh her off, for example. That's the point of an anti-unit, they excel at combating the one unit they have the advantage against but are awful against others. Mathilda and her Ridersbane are phenomenal against cavalry. Not nearly as much against flier. Bows destroy fliers, not so much Armored. Hammers destroy Armor, not so easily Infantry. That's Reina's weakness, she can't reliably combat anything outside of mages, and you said it yourself: it's silly to run all-magic teams. Halloween Sakura, a unit that exists to wipe out mages with low-damage shuriken, would be critical to them. And she already exists.

The situational heal is Lifetaker, not Amaterasu. It procs only in the event of a kill, which in all likelihood will only be against mages unless you specifically set up the unit for her to kill. Realistically, we're looking at it procing once, MAYBE twice in any given battle against any team worth it's ranking unless you're using her as your only means of offense, but you'll have a bad time trying that. That's ignoring the general AI gameplay, because AIs are not smart and randomly generated.

As for Amaterasu, you're greatly overstating its healing ability. You talk about it as if it heals every turn, it doesn't. It activates once for 10HP every two turns, just like Renewal does, only it doesn't affect the wielder in question. With her mobility as a flier, it is much easier for her to become separated from the group, especially on more terrain-heavy maps. You *could* try using that as your only heal, but it won't go over so well for you. Honestly, having a healer like Elise with Rehabilitate or Physic would help wall off a choke point better than Reina could. Staff-users can heal for more HP every turn, especially if they have Heavenly Light, which heals for 10HP map-wide. Reina gets 10 every 2, max.

Mages may be a large number, but they're far from the only demographic in the game. They're also not the only threat. As I said, Bows or any competent physical unit would completely shred Reina, given her HP, SPD, and DEF. Cavaliers, widely considered one of the biggest potential threats, given their range, can both outrange and outdamage Reina, putting her down with ease. As for the mages you listed, they share the exact same disadvantage against Sakura's Kitty Paddle, low DEF and a bonus against their weapons. Even if you don't keep it on Sakura, passing it to a high-leve, Mage threat like Felicia will put down virtually any mage. Thing is, just like every other threat in this game, planning for them and fighting them appropriately is the key. Of course you don't throw a mage at a mage-killer, it doesn't matter who. Just like you don't throw a Cavalier at the Ridersbane or Armor at a Dragon. You fight them with your best counter to them. In this case, literally any unit that's not a mage. Again, as you said earlier, an all-mage team is a dumb move to begin with. If your team is competent, you would have two, maybe three units that could eqsily handle her. Or, alternatively, the Raven Adept combination still puts a large damper on her output. Yes, she still gets a bonus against Tomes, but the bonus isn't enough to overpower the combination. This coming from someone who uses Raven Adept on S!Camilla, S!Kamui, and Cecilia (who can easily take the Ridersbane with it). Yeah, you could give her Cancel Affinity, though doing that negates your complaint about Lifetaker since the two can't be used together.

The same could be said of any unit in the game. Ayra is a danger to anything unarmored. B!Lyn is a danger to anything with wings. The entire point of this game is to build a team with units you enjoy that can cover each other's weaknesses. That's how building an effective team works. Using a powerful mage? Make sure to pack someone who can counter Reina. Again, virtually any non-mage unit can with a good amount of ease. Using a low-Res nuker? Pack a counter against mages.

I wouldn't say a Brave Bow build wouldn't work, it would just change her target.

Her utility is much more limited than you seem to believe. Again against anything but a mage she's quite vulnerable, same with Felicia, and her Amaterasu doesn't proc every turn. Basically any damage dealer can outpace it, really, but the top-tier units especially. Yeah, she's a threat to mages. She wouldn't be the only one in the game to be. If your team centers around your mage, then a good team would be prepared to deal with a mage-counter. With Lifetaker procing only on kills, her only real benefits are the Spellbane and Amaterasu. Anti-mage and healing, literally nothing that can't already be done with H!Sakura and Breath of Life. The only real difference is the trigger of their heals and the range. In the same number of turns, Sakura can heal more HP, but Reina has a slightly larger range for it.

As for the whole "with her kit and the other 75% of her team," that argument can apply to literally any powerful unit in the game. Sure, Reinhardt can easily wipe most low-mid-RES units. That's why you bring someone to cover for it. Reina a threat to your team? Like most any flier, pack a bow. Or a half-decent melee (AKA 50% of the cast).