Lightning reflexes, strategic thinking a bit of planning and occasionally brute power to overcome your enemy.
Unfortunately the number of cards(spells in the game) is not more than 70.
The five colors are remarkably well-balanced, each having a unique gameplay(stealing enemy mana/creatures, counter spells, inflicting direct damage to enemy hero, summoning mighty land/air creatures, mana gathering acceleration...)
Before dueling, players can edit their spellbooks mixing different spells and tactics.
You have to be strategic in planning what kind of attacks you want to use but there are so many variables to consider: build up the mana you need, consider the enemy's mana status, know the attacks they're vulnerable to, decide when to trade offense for defense...
It`s so hard to find such dynamic tactical games(with decent graphics and well crafted gameplay) nowadays : )