Increasing your Health Points is the best way to improve FS's damage, since strenght does little damage (only 40 times your Str points, not your Atk!) compared to the other component of the formula (dealing 80% of your resting HP as damage)
You should get equipments which increase your maximum HP% (Peco peco, green ferus cards, necklaces and spore cards, or even getting the Goibne's Set if you are able to do so), and strictly having your Vitality stat to the top.
The weaponry is extremely simple, you can use a Knife (yes, even a crappy knife), a Main Gauche or whatever other 4 slotted ninja-equipable weapon you can think of, as the Atk stat has no effect on the final damage. Just get 3 racial and 1 element or size damage modifier cards to get the best damage of this skill.
And be sure of having always a finger over your 'HP recovery items' key, or else youll sign your death sentence for being wandering around with 1 hp, when even a 5 int acolyte can kill you in a single Holy light cast.