Stay as far back as you can, and you may want to assign Magikal Attack Level 1 and Level 3 as quick spells. Start by dodging its long ranged attacks, then you will notice a pause. Run about halfway up to it, then start the Level 1 Magik Attack. The hit will only connect if the Black Guardian is glowing. Do this three times and it will draw you closer by moving the barrier. It is however still far enough to stay out of range of its short attacks. It will summon three Zombies and then recharge. As soon as your sanity drops for the first time, begin the Level 3 Magik Attack. It should connect just in time. Do this three times and it will draw you still closer. This one is difficult because you have to keep running. Dodge his attacks six times, and he will become winded and have to recharge. Use this time to let loose a Level 1 Magik Attack. Do this three times and he should be down for the count. Retrieve the essence, and you are done.