Bones of Steel

This spell greatly strengthens the bones of animated skeletons, literally making them as hard as steel and able to resist far greater amounts of damage.

Casting bones of steel on a skeleton will grant a +2 bonus to its armour class. The spell will stay in effect until the skeleton is destroyed, or be dispelled.

Regardless of the amount of times this spell is cast you may only strengthen the bones in a number of skeletons equal to your spell-casting modifier.