Idunn

This is an enemy phase focus build.
If you have supported Idunn with S support, things get even better.
Pivot gives Idunn more mobility and can block enemies from targeting a unit.
With Fort. Def/Res 3 you've got yourself a tanky armored unit with a decent atk stat.
Renewal gives Idunn self-healing and makes her annoying to kill as she also has pretty high defensive stats.
Atk Smoke 3 can give foes a harder time as a lower atk stat means harder to kill in one shot.
Distant def gives her even more defense against units such as Micaiah, who has an "effective against armored and cavalry" ranged weapon.
This does not mean she will never die, as Roy is a danger to all dragons. It's better to kill units that may be a threat to Idunn first during the player phase so that they won't be a nuisance later on, or make them target Idunn and fall into the depths of flaming hell.
High defense=more atk with bonfire.
This may not be the best build but I tried and it works fine.

I never thought I'd utter the phrase "Wow, what a bulky beauty!". Truly, we live in exciting times. Let's make a Wall unit! This build is really simple. Swap out Vengeful Fighter in favor of Wary Fighter, and give her either Close def or Distant Def seal. Done.

Okay, okay, you probably want a more in-depth explanation. I shall oblige.

First, Assets and Flaws. Any Asset will be valuable, except for Spd, and obviously Spd is the best Flaw for her, especially with this build. There's not much else to say here.

Next, her weapon. Obviously, her Prf is what we're going with. It gives her more Def, nullifies stat penalties, AND if her health is below 100%, or if she has a status effect applied to her(Panic, Isolation, Flash, etc), she gets a nice +4/+4/+4/+4 in-combat boost. It also gives her extra effectiveness against Armor units. It provides her with so many benefits with a condition that is ridiculously easy to meet, nothing else really compares. It basically stops foes from trying to weaken her, AND makes her stronger in literally every way.

Pivot is hands down the BEST movement Assist for Armor units who can't make use of the Armored Boots seal(as there is only one available, and Idunn's weapon doesn't mesh with it anyway) or Armor March 3. It's also great for dedicated Walls. No-brainer. Though if you can't get it right away, Swap can be used as a lesser version of the same thing.

Bonfire is excellent on Idunn as her Def is incredible, especially with her A skill. I tried Escutcheon and even Sacred Cowl to improve her wall potential, but they were essentially useless, so Bonfire will do. Moonbow and Glimmer are viable as well, since she won't be able to speed up her Specials here.

Idunn's defenses are incredible, especially with her native Fort. Def/Res 3. Distant Counter is great and all, but she loses a lot of wall potential if she doesn't make use of her base A skill. We're using her as a dedicated wall in this build, so we'll have to forego the obvious DC in favor of extra bulk. And thank goodness, that's a rare skill. Another great alternative is Brazen Def/Res 3 as it gives her an extra +1 to both Def and Res if her HP gets low enough, without the modest cut in Atk. However, her native Fortress skill is always in effect, which is a plus in and of itself. It's up to you.

Wary Fighter is self-explanatory. Vengeful Fighter is fantastic, don't get me wrong, but for true Wall tactics, Wary is the way to go. Once she gets hurt at all, she takes even less damage, so there's really no reason to worry about getting below Wary's threshold in most scenarios.

I kept Ward Dragons as I put her on a full Dragon team. It has proven extremely useful. If you're adding her to any other team composition, Atk Smoke, Ward/Goad Armor or Armor March(if in an Armor team, obviously), Tactics, etc will do just fine. Ploys are also viable with her Res hitting 40 at base if she's got Fort. Def/Res equipped.

I tried Close AND Distant Def seals, and I'm happy to say that both are great. I ended up giving Distant Def to another unit who proved to need it more.

Idunn has top-notch Wall potential, easily comparable to Surtr, though she focuses on avoiding most stat debuffs and enriching herself with stack-friendly in-combat boosts, rather than mirroring Surtr's methods of punishing with out-of-combat methods, like proximity damage and applying stat bonuses(which can be panicked in some cases) and penalties. In a way, she may be better for the role, especially with her higher Res, penalty immunity, and ability to appreciate both Dragon AND Armor-exclusive buffs. Despite her delicate features and petite frame, Idunn stands tall and mighty.

This build is an absolute force to be reckoned with. I took off Vengeful Fighter and replaced it with Bold Fighter and gave her Quick Riposte in her S-Slot to compensate for Vengeful Fighter, that way she doubles both attacking and defending. Then I gave her Armor March for her and her other armored con rads to get around quicker. Since Bold Fighter makes the cool down on the special go by faster, it's better to replace Bonfire to something better. Aether is the way to go that way she does increase damage and heals herself making her survivability ridiculous. Idunn is one of the best armors in the game, and this build just increases that.

Really powerful on enemy phase, Idunn relies on her strong base attack and wary fighter to make sure she takes little and deals a lot in return. Make sure her health is up, cus her speed is bad. Wary fighter takes care of this, and with Idunn's sky high def/res, Idunn makes a really strong enemy phase breaker.

I've chosen Ignis because of her massive defense - and she can take multiple hits from enemies, making up for the special cooldown charge of Ignis. She's got armored boots because I don't have any units with armor march. I've no idea why I gave her Breath of Life. Fun Fact - she's +1 because I managed to pull 2 in a row in 1 summoning session.