The accuracy of your spells is determined by your main-handed weapon's skill I believe.
For heavy strike I use:
My set Aint perfect though, but it gives extra accuracy to reduce the number of whiffs I get whilst maintaining some damage~
Reason there's no Str earrings in there is because of inventory issues, but feel free to use them.
Could probably swap the head for Aia's bonnet if DEX attributes to Physical spell accuracy too if you wanted.