As with the Soul Master, this fight isn't actually all that bad.
Having Descending Dark helps greatly, as the damage of that spell is quite frankly insane (especially when coupled with the Shaman Stone), plus the added i-frames help immensely for dodging damage. It's not required though.
Using Descending Dark above the Tyrant when he's in the air will maximize your damage, and if you have Shaman Stone you can do upwards of 88 damage in one hit (the boss has 1,250 health total).
He re-uses most of the attacks from the Soul Master fight. The only real difference from the normal one is that he attacks slightly faster, and his orb surrounding attack has 6 instead of 4 orbs.
Defeating him will award you with 300 Essence, and this achievement when done for the first time.
Finally - as with all bosses - memorizing the attack pattern (or at least knowing the various attacks and their telegraphs) helps greatly since it lets you plan ahead.