Spring Sharena +atk -res or +spd - res

Speed, you can run sweep skills to cheap dragons or armor without fear.

I suggest +spd, but arguably +atk if you compete with merged units and want to play offensively because she's not doubling the fastest units in the game either way. For PvE, probably +spd except on abyssal maps, but for those you probably use other units.

I would go with speed, because then ahe is fast enough to double most units and avoid doubles from someof the faster units. Plus if you are using her Swift Stance 2 that further protects her from getting doubled on the wnemy phase and covers for the RES bane. Attack is good, but in my oppinion, that is only if you are running a blade tomb.