Beruka

FD2 because I haven't bothered upgrading it yet

Basically Beruka is a tank that will occasionally proc Ignis for upwards of 42 damage not counting flier buffs. Yeah her attack is garbage and so is her speed and resistance but I'm super committed to this niche build on an even niche-r unit

Flier tanks that aren't premium units like Myrrh cannot operate in the same way that an infantry or armor tank does. Specifically, it is difficult for fliers to achieve a decent enough degree of mixed DEF and RES to justify giving them Distant Counter. Obviously, if that was the goal in mind, you simply would just use Myrrh, or opt for a mixed team with something like Sheena.

Depending on the map terrain, however, you will sometimes have no choice but to rely on a flying unit to act as a physical tank, and not necessarily secure a kill. In other words, the unit you want is one that has the staying power to use assist skills and deliver Goads, Drives, etc.

Although it is questionable enough to upgrade a seal for Fortress Def 3 with 150 coins, you may be hard pressed equip Beruka with Close Def 3 as opposed to your core units. Therefore, it seems reasonable to go for a double dip on Fortress Def 3, since the unit archetype you are aiming for is one that isn't aggressive. The worst case scenario is you can always just use this seal for other units, because it essentially turns them into dedicated Ignis bombers.

QR3 comes without argument, but the cost, again, may be questionable. It is arguably Beruka's best B skill, so the choice to settle for the "QR2 of shame" is up to you.

As a support-archetype tank, C skills should always be flier support skills. Hone Fliers is expensive, but one or more Goad Fliers will stack with one instance of Hone. It goes without saying, it's easy to inherit (from Palla).

A Beruka like this operates well with player phase fliers. Caeda maneuvers easily into kills on armors and frail mages, but may need help against blue tanks. As the unassailable flier tank, Beruka is a great teammate for Caeda, another strong budget flier.

Truly strong bow users can still be problematic. Normally you would use Distant Counter to retaliate, but you have to drop Fortress Defense for Iote's Shield. Or, you run Iote's Shield without DC. Needless to say, you are better off using a TA Raventome flier in your team, as they might additionally be able to control other problematic colors.

This build expects that you really want to invest in Beruka. I'm basing it off of my 5*+6 with Summoner Support, so your stats may be different from mine.

Skills:
Slaying Axe (+Spd): A good overall weapon that helps boost her speed and hp a bit more, and allows for 1 charge Glimmers
Reposition: It's pretty self explanatory, but you can use whatever you want for the support slot.
Glimmer: Since she already comes with it and can charge it turn 1, as well as how much her ATK will be buffed, it's a good skill for her.
Life and Death 3: This is where it gets fun. +5 Atk/Spd is what we really want for this build, and it doesn't hurt he physical bulk that much. Mages were already an issue, so reducing her Res doesn't hurt us much. You can swap this for Fury 3 if you want to keep her bulk while letting her get into Desperation range more easily.
Desperation 3: With the buffs she's getting, (and assuming yours is the same as mine) she'll have a respectable 38 speed before buffs. Trust me, we can get it higher.
Threaten Speed 3: Something to artificially make her faster. Even then, C slot is flexible.
Speed +3 Seal: Just to get her to that 38 speed.

This build may seem odd at first, but once you start adding in other units, she will decimate anybody who stands in her way. I already did the math for when she has these buffs:
Hone Fliers
Camilla's Axe (+Eff)
Goad Fliers
Ally Support w/ Corrin (M)
Yato (+Eff)
Spur Speed 3
Drive Attack 2

With these buffs, her Attack comes out to 77, with a speed of 61 (assuming I did it all right). For someone with such low base speed, that's amazing. If you wish to try it, go ahead. Your stats might be more or less depending on investment and skills.

Beruka is a good 4*+10 project since she's so available in the green pool and she is a very good tank. She's the tankiest flier and one of the tankiest units, while with her attack stat she hits like a wet noodle, we rely on her specials to actually deal damage.

Weapon: Slaying Axe+ (+Defense). We buff her HP and Defense with this refine, while speeding up her Special.
Assist: Swap. If Beruka doesn't activate her special and kill the enemy in her follow-up attack, she can use Swap to trade places with her allies to finish them off.
Special: Ignis. A 4 cooldown special, that with her weapon turns into a 3 cooldown. With her low speed comboed with Quick Riposte, Beruka should be able to have her special charged every turn.
A skill: Fortress Def. This might not be the ideal skill, but sacrificing her already low attack for better defense so she hits harder with her special is the idea here.
B skill: Quick Riposte. Beruka's speed is so low she'd never double anyone, Quick Riposte is chosen here to allow her to charge her special faster.
C skill: Fortify Fliers. Defensive buffs for her flier friends.
Seal: Fortress Def. Same as her A skill!

Basic flying tank build, using both pricier skills and some that are more budget. Given Beruka's innate tankiness, she doesn't really need Hone Fliers to do damage, and is better off buffing speedier allies.

Fortress Def gives the biggest bang for her buck, and Ignis scales well off of this increase in Def, with Ignis adding 36 damage. Guard prevents any enemies from activating their special so long as Beruka keeps her health up- a feat that's rather easy against blue and green units.

Iote's Shield seal prevents a large amount of damage from archers, though Merric will still very likely eat through her HP by using magic damage.