Gunnthrá

Gunnthra is a really cool hero, and I got her at +spd -def.
While the page says otherwise, +Spd is actually a really good boon. With all the speed buffs she will be able to double even the fastest units in the game.

Atk/Spd solo is STILL not in the unit builder, so I have atk/Spd bond. (seriously, the unit builder really needs to be cleaned up, it is such a mess.)
While bond still can work well, the solo is the best one especially because she is Calvary and can outrun her teammates.
Her attack will reach sky high with debuffs (make sure to have someone with chill res on your team too), so heavy blade is the best option here. With heavy blade, glimmer is also great. Since M oonbow isn't that great when you are debuffing your opponent and with gunnthra's attack being really high with all the debuffs, glimmer will ensure her special proc for a lot of damage.

The wave skills are also great because she is self reliant. I opted for attack wave, but speed wave works just as well too.

I like how the res boon balances her hp bane (so that she's still in the 145-150 bst bin). Also helps her ploy other units.
Base kit is great, although I don't like fortress res 3 on her as her base attack isn't high to begin with. So fury 3 comes into place, bumping up all her stats by 3 (with the exception of hp).
Seal is flexible.

Mine is +atk. So I took advantage of her good atk and put death blow. her res isn't so good but I still kept her c skill since I was too lazy to find a new skill to put on her. With having death blow and her B exclusive skill it, allows her to do more damage and have a bigger advantage.

This build ultilize the power of her weapon to their maximum potential by applying as much debuffs as possible to one enemy, resulting a whooping +22 damage bonus from Blizzard if she pulls off Chilling Seal with Res/Def Ploy 3. Resistance boon is pretty much a must as she has to have high Res in order to activate ploys, even on a high Resistance unit.
Special: Glimmer (with her debuffs, glimmer will do a wonderful amount of damage with +22 damage if she pulls off all debuffs)
A: Fury 3/Fortress Res 3 (Fury 3 is much better but if you do not have fury fodder you could use fortress res as a budget A passive)
B: Chilling Seal
C: Res Ploy 3
S: Def Ploy 3
The next time you don’t pull a +Atk Gunnthrá, this is a much better alternative if you want minimal investment but still want to use her on your team. Good luck summoners!

[Seal: Resistance +3]

Summoning on Legendary banners is the worst and I'd never wish it upon anyone; this build is just for the people who happen to have a +Res -Def/-HP Gunnthra (although +Atk and +Spd and no -Res bane also work for this, they just get ploys off less).

Between Blizzard, a superboon, and the Res +3 seal, even with the Life and Death penalty, Gunnthra still has plenty of Res to get off ploys, which then boosts her offensive potential with Blizzard. Swift Sparrow can be taken over Life and Death, but it makes Gunnthra less offensively potent and more defensive. Both of these options, however, are infinitely more preferable than Fury, which actively works against Gunnthra's prf chill skill. Don't put Fury on Gunnthra if you have Chilling Seal as her B.

If running Life and Death, Draconic Aura is marginally better than Iceberg. If running Swift Sparrow, then Iceberg should be taken instead.