Gunnthrá

Rather than going for a special that requires a high particular defensive stat, this build focuses on the weapon's bonus attack properties.
Blizzard's effect adds, take note, more attack than bonus damage dealt.

Let me talk about the B skill first since its an exclusive skill just for her. Blizzard's synergy with Chilling seal is so good that it shouldn't be changed at all because it can potentially give 12 more damage to an affected target, increases the chance for Gunnthrá to double the enemy and lessen the damage taken on counterattacks due to the debuff so please never take this skill off her.
This can be used to her advantage in many ways. May it be via her strength or even as a supportive skill.

For A skill it's either Fury 3 or Swift Sparrow 2.
I prioritize Fury 3 because it will make her a better unit herself. The stat overall since she is a very fragile hero, adds more bulk to her, increases her damage and gives her more chance at doubling enemy targets.
The only time I'd like Swift Sparrow 2 for her is when she has a team that is built around her and can support her the whole entire battle.

And as for the C skill I think the default skill Res Ploy 3 is best overall.
The fact that she can debuff the enemy once again if she triggers makes it even scarier. Having this would give her a potential of 10 more damage to the enemy target.
Cavalry buffs are also a good option if you are running a cavalry oriented team.

Special will be, without a doubt, Dragon Fang. 50% of your total attack when triggered? Completely synergises with the total attack power of Blizzard's potential. Literally deletes any target Hero with debuffs.

For Sacred Seal, it has got to be Heavy Blade 3. Faster cooldown on attack if your attack power is higher than the opponent making you activate your Dragon Fang Special faster. Nothing much to worry about since you can delete pretty much anyone as long as they are debuffed.

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Also, please take note that Gunnthrá is a Hero that is hard to use because she requires debuffing the enemy to work well so please be careful on making moves, plan ahead and practice on using her advantages.

This is a straightforward build but how she functions is very different, her tome acts like a reversed Blade tome where her damage is increased the more debuffs her target suffers.

Fury 3 is a nice option however you must take cautious as it knocks her out of her B slot skill's effect that only takes action above 50% of her HP.

One the best optimal builds for Gunnthrá. This is good for both Horse Emblem and mixed teams. Gunnthrá’s a powerful debuffer, the best Green horse mage now in the game. This build optimazies on her ability on debuffing and killing.

This Build aims to get the maximum damage output with Gunnthrá's unique Blizzard tome. With a Res boon + Fortress Res 3 and Blizzard she hits a respectable 37 Resistance, allowing her to proc Res Ploy even better.
At the start of your turn, if Gunnthrá has ≥ 50% HP, one enemy (or more, if they have the same amount of Def) gets hit with a nice -6 Atk and -6 Spd, which grants her extra damage with Blizzard. And after she attacked, the enemies near her will get hit with -7 Atk, and maybe also Res Ploy.
You can also run Fury if you want more Atk and Spd, but i personally don't like the recoil damage since it clashes with her Chilling Seal.
Distant Defense is also a nice option if you want to check Reinhardt or other blue and green mages.
So, as you can see, she is a really good debuffer and has nice nuking potential, if she can hit someone who is debuffed. If you pair her with Arvis, Saizo (with refined Smoke Dagger+, which debuffs ALL of the foes stats and enemies within 2 spaces for -6) or any other good Ploy or Panic User, she can dominate the battlefield with her good mobility and wide reach.

Res is probably the strongest boon for her base kit, granting a +4. Fury will help bulk that up as well as boosting spd and atk. A -def will help keep her magic bulk intact while drawing away enemy Gunnthra's debuffs away from your bow lyns or The reinheart