|Of the Askr trio, Sharena has the most balanced stats and occupies a solid color in the current meta. Therefore, it is not uncommon for her to earn a spot on players' teams even during the weeks where she is not a bonus unit. However, as with most units that lack IVs and/or can't be merged, Sharena starts falling behind rather quickly when you start facing more difficult opponents in arena. She is especially helpless against the cavalry due to not having the bulk to take hits or the offensive capability to neutralize them. But, don't let this discourage you from building Sharena as she can still be a very useful unit, especially as an easily accessible bonus unit for 1/3 of arena seasons.
Her native weapon, '"`UNIQ--item-351--QINU`"', is a legendary weapon with Threaten Attack 2 built in. With its 16 might and above average passive that helps bolster her survivability, there is little reason to change her weapon.
For her A slot, '"`UNIQ--item-354--QINU`"' is a good option, as it is on many units, due to the +3 to all stats that comes at the relatively small price of 6 non-lethal damage after each round of combat. Since Sharena's base stats are rather average to begin with, this boost is greatly appreciated in each area. Most notably, it brings her speed stat to 35, which if further shored up by a speed buff, will allow her to avoid being doubled by fast units who typically run a speed boon. However, if these enemy units run speed boosting skills and/or have speed buffs also, they may nonetheless double Sharena, so take caution. '"`UNIQ--item-357--QINU`"' is another good choice since it greatly mitigates damage from Red sources, allowing her to be of greater use past her first round of combat. The damage increase is also appreciated, but is still insufficient to kill common Red units like Eirika or Ryoma without an attack buff. Unfortunately, tankier Red units such as Ike cannot be killed even with an attack buff. Nevertheless, the damage mitigation against Red enemies is still enough of a benefit to warrant Triangle Adept being one of her best A slot skills. Just remember to keep her far away from Green units.
'"`UNIQ--item-360--QINU`"' in her B slot allows Sharena to remedy her original inability to kill sword units by guaranteeing a double attack. This allows her to become an effective counter against sword units and prevents the target's allies from activating Wings of Mercy if applicable. The ability to completely ignore a specific unit type, especially one that is omnipresent in arena, should definitely not be underestimated as it allows you to focus your other units on dealing with other threats.
As with most units, Sharena's C slot is flexible and should be based on whatever her team needs the most. Typically, this is a buff of your choice.
Because Sharena is a light combat unit who cannot stay in the thick of battle for too long, she needs a special that can activate as quickly as possible to help her win duels. Therefore, '"`UNIQ--item-363--QINU`"' is the best option with its 2 charge cooldown. Against any unit she doubles, she is guaranteed to activate it, further allowing her to secure important duels.
'"`UNIQ--item-366--QINU`"' is the best assist that allows her to be relevant even if she is too hurt to continue fighting. It allows her to help position her allies better for engagements or move them to safety.
For the Sacred Seal slot, '"`UNIQ--item-369--QINU`"' or '"`UNIQ--item-372--QINU`"' are her go-to choices for combat. More speed is always welcome, especially since one point of speed can be the difference between doubling a unit or getting doubled. '"`UNIQ--item-375--QINU`"' gives her additional bulk, which is appreciated since she is a melee unit who must be up close and personal to fight. '"`UNIQ--item-378--QINU`"' is an acceptable alternative, although it is not likely to make much of a difference in duels.
As one of the starter trio you receive and a bonus unit for 1/3 of arena seasons, Sharena is a unit that should be built by everyone, especially for those who cannot afford to pull on each new banner for the newest units. She is most definitely serviceable and can fit in well into just about any team.