Which refine should I use?

I'd go with speed because it throws you up to 38 and acts as a secondary form of bulk to help her deal with retaliation hits, while also bolstering her offensive capabilities by giving you a solid 44 on player phase.

Either go for speed or resistance refine,

Go for the speed refine if you want her to double more units with her 35 spd with darting blow she will double most units (but maybe not ayra or Mia)

Go for the res refine if you want her to survive mage attacks and deal more damage with Glacies, with her res she can kill most Non green units with glacies and survive most non-green mages.

If you want the atk refine I suggest not to cause it's better to double mosr units than attack once on already fast units but attack is better in some situations.