It honestly isn't, at least in the beginning.
Magic attacks will drain your stamina bar. Additionally, you will build up fatigue, capping your stamina bar at a lower level over time. If it gets too far to the left, somewhere in the middle, you'll be unable to cast a spell and need one of the phial items to recover.
There's also the Focus stat, which I haven't quite figured out. You need a certain amount of focus to be able to cast higher level spells and Focus goes down with Fatigue as well. I don't know if Focus or Stamina level influence your ability to cast spells, or even both, yet.
Basically, you'll be running light gear, a fast weapon for self defense/mobs, and just spam magic attacks. Works especially well on bosses, since you can stay out of their range and cheese them a bit.