There are two ways of defending yourself from a magical attack, depending on whether the caster is just trying to physically harm you vs. trying to affect you in some other way. The former requires a Defense roll. The latter requires a resist roll.

The Defense Roll
If the caster is just trying to use magic to cause you damage, you make a normal Defense roll, just as you would against a sword strike. The potential damage you need to defend against is calculated by taking the spell's Base Difficulty plus the caster's Primary Attribute (see Tradition descriptions) plus any extra successes the caster has rolled and has decided to put into additional damage (rather than deciding to use them to buy off Burn or otherwise enhance the spell).

So, for example, Menelarus (page 110) uses his air elemental magic to send an arrow of air at his opponent. He could have used fire to hurt his opponent -- either way, he is trying to directly cause physical damage with magic, so the target gets to use his Defense.

As an Elementalist, Menelarus has to cojole the elements into doing his bidding. The Primary Attribute for Elemental casters is Charisma. Menelarus has a Charisma of 3. Because he's trying to do damage, the Base Difficulty of the spell is 2. He has a Magic Skill Rating of 7, so he can roll up to 7 dice. He decides to roll 4 dice and gets 3 successes. He takes 1 point of Burn. He could use his extra success to negate the point of Burn, but he decides to add it into the spell's damage.

So the air arrow spell damage is 2 (Base Difficulty) + 3 (Primary Attribute) + 1 (Extra Success) for a total of 6.

Say the target has a Defense of 6. He could take his Average and negate 3 of the 6 points of Damage (reducing his Health by 3) or he could try to roll six dice.

Note that is Menelarus tried to cast something more difficult that still only caused physical damage, the Base Difficulty would still be 2. For example, if he wanted to hurt two people with air arrows, the Base Difficulty (which is used to calculate spell damage) would still be 2. However, it's more difficult to hurt two people with one attack, so the Final Difficulty of the spell (which the caster has to roll to have the spell work as intended) is 3. Just because a spell is more difficult to cast (larger area of effect, multiple targets, longer range, more complicated effect, etc.) doesn't mean it should cause more damage.

So using the same roll above, but against two attackers would allow Menelarus' spell to go off as intended because he rolled 3 successes. He would get no extra successes, because the spell difficulty was 3. An air arrow would fly at each of the two opponents. They would Defend against it like so:

2 (Base Difficulty) + 3 (Primary Attribute) + 0 (Extra Successes) for a total of 5. Both targets would need to Defend against the 5 potential points of damage coming their way.

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The Resist Roll
Sometimes being physically tough and quick or well-armored cannot help against magical attacks. That's when you have to resist a spell.

Say Keke (page 114) wants to paralyze her opponent using Animism magic.

It's a Base Difficulty 3 (affecting others in a strong, but uncomplicated way -- page 132). She decides to roll 6 of the 9 dice she has in her Magic skill. She gets 4 successes. The spell goes off as intended, and she decides to use the extra success to make the spell more potent, rather than buying off one of the 2 point of Burn she took when she cast the spell.

The opponent gets to resist the Spell Potency, which is calculated like so:
3 (Base Difficulty) + 4 (Primary Attribute) = 7. Divide that by 2, which equals 3 plus roll one die to see if it's 4. We'll say the roll was a failure, so the Potency so far is 3. Then add the 1 extra success for a total Spell Potency of 4.

The opponent gets to attempt to resist the spell potency of 4. The GM decides that the appropriate Attribute to resist paralyzation with is Willpower. The target has a Willpower of 3. He rolls double his Willpower Attribute (6 dice) and gets 4 successes. He resists the spell and is not paralyzed.

Note: Most of the time, Willpower can be used to resist spells. However, the GM could just as easily said resisting paralyzation is a Body roll or Strength roll -- the GM could make a case for either. Just like all other GM judgment calls, the key is to be consistent.

Hope that helps.

Exactly what I needed - thank you!