The Citadel of Chaos

You slash about madly with your sword but cannot hit the creature.
Either it is extremely quick, or it has no solid body for you to hit!
Its teeth are now tearing at your flesh and you can feel blood on your leg.
You will have to protect yourself with your magic,
or face certain death from this unseen creature.

Will you cast a Strength Spell,

a Weakness Spell,

or, if you cannot or will not cast either.