I'm an EK, so I can tell by my own experience.
When you're using full def, you will deal significantly less damage, and you will take longer to kill the creatures. Also, if you only attack with spells, you'll spend more mana and consequently will use more mana potions. Speaking about your imbuements, your critical hits will deal less damage and you will recieve less mana and life back when you attack a creature.
So, your best option when hunting is always use full attack, you will deal more damage, kill creatures faster deal more damage with critical and receive more mana and life back from your imbuements.