Would Steady or Fierce Breath be better on Fjorm?

I prefer steady tbh. You can never have too much defense when using her as a bonus unit in arena and facing green opponents. Attack helps but, with enough support from the other units she should still be able to kill, she can't kill if she is dead...

This depends on what you want your Fjorm to do.

If you stick with her default kit of Shield Pulse + Ice Mirror, I think that Fierce Breath is a really good option for turning her into an anti-Ranged unit. Ice Mirror does add the reduced damage to her own damage, and the extra attack when she's attacked is never a bad thing to make sure she hits back hard. (Steady Breath actually isn't the best choice here since it gives her more Def, making it so that she takes less damage from physical sources, which in turn means she deals less damage with her Special procs.) If she kills on the retaliation, then she'll have Ice Mirror ready for the next fight. If you run the Quick Riposte seal, you could bait faster and bulkier Ranged units; she can proc Ice Mirror up to two times in that round of combat, most likely killing whoever was attacking her, and still have it ready for the next fight.

Another common, albiet expensive, set for Fjorm generally has her running Aether + Quick Riposte (in the B-Slot rather than seal slot) for damage and sustainability. For a build like this, I would recommend Steady Breath over Fierce Breath since she wants to take as little damage as possible. As weird as this may sound, she would often heal more damage off of an Aether proc if she ran Steady Breath over Fierce Breath since her initial hit back would be doing less damage in the first hit, which leaves more HP left for her to potentially "steal" when she hits back the second time. (When combined with the extra bulk she gets with Steady Breath, she will probably end up with more HP after the fight than if she ran Fierce Breath.)

Do you care about arena? If so, keep your Tiki for possible future merges. She is in a league of her own when it comes to arena scoring.

She is also pretty much IV proof, so no matter what her nature is she will still outperform 90% of units.

I'd go with Steady Breath. Mostly because tiki is much harder to pull, and has way more value as a unit. I do believe Fierce Breath is the generally better skill for her, though. Fjorm can get enough defense against greens in Arena, whereas attack is a real problem. If you face greens like Hector with 47 defense unbuffed, you need 60 attack JUST TO DEAL DAMAGE. There are ways to circumvent, but still, generally speaking, I would take attack. Also, Steady Breath does nothing against Grima except for boosting special cooldown.

I guess it mainly depends on what you plan to do with her? If you're keeping her ice mirror/shield pulse build then honestly you'd be better off giving her a stance skill instead, since she won't benefit from the extra special activations breath skills offer her. then save the breath skills for someone who can make better use of them.

That said if you''re doing a different build then I'd pick steady breath. Fjorm would be relying on special activations to do the bulk of her damage anyway so she'd appreciate the extra survivability steady breath offers her, giving her more opportunities to activate said specials & whatnot

For general use, I think I prefer the extra atk, but she could use more of any stat, really, she is so generalized

If you're using a damaging special, then definitely Steady Breath. She's a tank. Tanks like points in defensive stats. Tanks are especially viable in Heroes due to offensive specials. If you feel like she needs the extra damage, damaging specials can easily make up for that, especially with the Breath skills' spam potential.

Fierce Breath is honestly kind of niche, only really useful on super tanks, tanks whose stats are so astronomically high that they don't need any more points in defensive stats.