Strength is good for Shale if you want her as a tank/off. Boost the stats for the role you want her to fill. But don't try leveling her. She really doesn't like being shrunk anymore than she is. Leave her upright.
Shale is extremely good against the Archdemon if she has her best spirit crystals on. It will make Allistair pretty angry, but my Shale managed to hold her own against AD very impressively.
I did initially put some points in Magic because I thought she would heal better. I guess that doesn't work.
32-38 con, rest in strength. Add a couple of points in dex if you feel like finishing off her warrior line, but disengage and perfect striking aren't necessarily worth it.
You need 38 constitution to equip brilliant large crystals. However the brilliant small natural crystal gives you +6, so if you're using one (or just swapping one in when equipping new large crystals) you don't need that extra constitution. Then the damage of her talents is based on strength so go with that. She also generally has enough stamina, especially if using a large fire crystal.