-Atk Robin

Sharena has Fortify Def and Rally Atk. Fortify Def would reduce Takumi damage by 4-8 or a bit more depending on the exact maths behind Robin's weapon advantage. Similarly, Rally Atk would give him a bit of a boost that would help secure Takumi kills.

Alternatively you can screw Rally Atk, send Sharena to Takumi's face and tack on some damage. She has the Def to survive a counterattack, and the Spd to double a -Spd Takumi. Then send Robin to clean up afterwards. She's obviously good at killing reds, and has the bulk to act as a buffer between Robin and enemy units. You can also have her simply attack someone else nearby, and Fensalir will automatically drop Takumi's Atk by a further 4 which can really neuter his damage when combined with Fortify Defense.

All in all she's a good support unit, handing out two very flexible yet useful buffs that can make up for M!Robin's poor IVs. She can tank hits somewhat so she's a good front-liner, stopping others from reach M!Robin. Aiding in Takumi kills is generally difficult (Due to his rangeless counterattack) without buffs, bulk or indirect debuffs, all of which she has.