So I'm gettin into staking a bit, mage boxing with smoke/shadow/blood barrage. I have 99 Magic, but is Magic and HP all that matters in a mage boxing duel? Or does defense come into play as well?

Defense is something like 30% of your magic your overall defense, if I remember correctly.

 

The accuracy of a combat spell is determined by a number of factors, including:

 

The caster's Magic level;

The caster's bonus to Magic attack;

The target's Magic and Defence levels;

The target's bonus to Magic defence;

Any Magic- or Defence-enhancing Prayers the caster and target might have active.

 

The Defence skill plays a role in Magic combat: Defence against spells is measured as 30% of the target's Defence level and 70% of their Magic level.

 

From RS Wiki