|When Fire Emblem Heroes was first released, Sheena was more or less universally ridiculed as a unit who could not accomplish anything. She was the closest thing to a tank that the game had to offer, but the dominance of Red units at the time and her inability to output any meaningful damage meant that she couldn't practically be used.
Fortunately, Skill Inheritance has given her the opportunity, albeit an expensive one, to become a dominating force on the battlefield by augmenting her already-impressive defenses while adding a punishingly strong offensive capability. Additionally, the decreased presence of Reds and the increased dominance of Blues and Greens mean that there are even less hard counters to Sheena in the arena. With Blue mages such as Reinhardt and Corrin (F) (Nohrian Summer) and Blue physical Brave weapon users such as Cordelia and Tana being common threats in the arena, all of which who are vastly empowered by movement type buffs, Sheena has finally found her place in the meta.
For her IVs, Sheena can make use out of any boon that isn't +Speed (as her Speed stat is too low to salvage apart from stacking three Goad Armor buffs, which doesn't synergize well with her role or with most Armor Emblem teams). However, her top two boons would arguably be +Defense and +Resistance, with the latter being slightly better. +Resistance allows her to mitigate magical damage slightly better to preserve her combat potential for future engagements. +Defense achieves the same purpose if the enemy team is more physical damage oriented while also providing a small boost to her Ignis/Bonfire damage.
Slaying Axe+ is the optimal weapon for Sheena as it is a 3 might upgrade from her native Killer Axe+. The charge reduction granted by Slaying Axe+ is critical to Sheena because it allows her to activate her special in a timely fashion.
Most armored units innately lack enemy phase capabilities and/or have poor player phase capabilities, with Sheena being no exception. Therefore, her optimal A slot skill should be Distant Counter, as it allows her to retaliate against ranged units who would otherwise attack her with impunity. This allows her to be a reliable check against some of the most oppressive threats in Arena, namely Reinhardt and Brave Bow archers other than Lyn (Brave Heroes).
For her B slot, Quick Riposte 3 is the natural choice since it allows her to perform a guaranteed second attack when initiated upon in her first round of combat (and successive rounds if the HP threshold is satisfied), an action that would otherwise be impossible for her due to her low speed as an armored unit. This second attack is also her opportunity to activate her special, which is the bulk of her offensive capability.
Her C slot is flexible, although Armor March 3 is a good choice if she is in an Armor Emblem team and needs to be the second carrier of the skill. Alternatively, she can run Ward Armor to help support her armored allies and make them even harder to kill. Goad Armor can be another alternative if using a more player phase oriented armor team. If Sheena is used outside of an Armor Emblem team, her C slot should a buff of your choice that best supports her team.
Sheena's relatively modest base attack of 30 is deceptive of her offensive potential, which rests entirely on her special that makes use of her high defense. Ignis is a better choice than Bonfire in most cases due to the Sheena's low speed that allows her to be doubled by virtually all enemy units and the attack sequence of Quick Riposte (on a non-brave weapon enemy phase, the order would be Enemy attack -> Sheena counter -> Enemy attack -> Sheena counter). As a result, Sheena will always have Ignis activated in the first round of combat on her second attack as long as she is attacked first, making it a superior choice over the weaker Bonfire, which will also activate on her second attack.
However, there are some cases where Bonfire is beneficial due to its lower charge timer, namely when the enemy has a Brave weapon and can quadruple attack Sheena to potentially kill her before the Ignis-powered second attack occurs. An example would be Cordelia (Bridal Blessings), who can survive a regular attack from Sheena but can quadruple attack her and kill her before allowing Quick Riposte to activate. Bonfire remedies this situation by allowing Sheena to strike back immediately on her first attack and neutralize her attacker.
Pivot or Swap are typically an armored unit's top two choices for an assist since they make the most of the one movement square restriction. Pivot allows for Sheena to pseudo-gain an extra square of movement if there is an ally she can "leapfrog" over while Swap allows her to switch positions with an ally to get into the thick of combat. This is especially useful on maps with choke points which make Reposition potentially unreliable.
As for Sacred Seals, Sheena can make great use out of the Atk Ploy 1 seal due to her unusually high resistance. This allows her to be even better at taking hits from the enemy. Distant Def 1 provides additional, guaranteed mitigation against ranged damage, which is typically the bulk of the source of damage she is receiving. HP +3 is also a valid choice on Sheena, as she is a tank who appreciates every bit of defensive boost.
All in all, Sheena is an impressive unit who can serve your team well with her unparalleled defensive capabilities while retaining the ability to deal significant damage. If you have the resources, you will not go wrong investing in building her.