This playstyle chart is somewhat of an approximation, but it should work well enough for most purposes.
Offense/Defense
On a scale of 1-10, how Offensive/Defensive is your spellbook?
Where 1=As offensive as possible, and 10=as defensive as possible. If you're having trouble answering this one, asking yourself how many rounds it normally takes your spellbook to win could point you in the right direction.
1 2 3 4 5 6 7 8 9 10
Essential Resources
What are your spellbook's essential resources? You need to have a sufficiently larger amount of your essential resource(s) than your opponent has in order to win.
1.) Mana
2.) Options (Flexibility/Predictability, also can be thought of as spell points or number of spells)
3.) Actions (including pseudo-actions)
4.) Favorable Positioning (Range/Los)
5.) Life after Damage
Sources of damage
How does your spellbook deal damage?
A.) Attack spells/one-use attacks
B.) Creature/conjuration attacks
C.) Direct Damage/Damage over time
How offensive or defensive a playstyle is overall is the sum of how offensive or defensive they are for each resource. And that is determined by the cards in your deck.
For instance, using the Meditation Amulet's ability is a defensive mana tactic. So is conjuring a mana flower.
Revealing an essence drain on an enemy creature is an offensive mana tactic.
Summoning a creature is a defensive action tactic. So is casting rouse the beast on a creature you just summoned.
Stunning or destroying an enemy creature is an offensive actions tactic.
Equipping yourself with Mordok's Tome or a Mage Wand a defensive options tactic.
Destroying your opponent's Wand of Healing is an offensive options tactic.
Force push on the enemy mage to bring them within range of your melee attack is an offensive positioning tactic.
Teleporting yourself away from a big enemy creature is a defensive positioning tactic. Since positioning is purely a relational resource, it is only offensive or defensive in terms of how it relates to other resources. Usually, lowering someone else's resources requires being in range of them, while increasing your own resources does not. Therefore, increasing the distance between your own things and opponent things is defensive positioning. Decreasing it is offensive.
Throwing a powerful attack spell at the enemy mage is an offensive life after damage tactic.
Casting a minor heal on yourself is a defensive life after damage tactic.
Edit: got offensive and defensive confused i think. Think it might be better to define "defensive" as increasing your own resources, and offensive as lowering opponents resources.