Zenith

This is a very easy boss. He only has three attacks that are all easy to deal with.

Attack 1: He cracks a big whip that causes rocks to fall from the ceiling.
Attack 2: He waves his head side to side calling some weak minions.
Attack 3: He puts his hands together and shoots a cone of fire. Shoryuken!

I don't want to give any spoilers so I won't go into detail too much about location. Once you go into the red portal (story related. You'll know it when you see it), equip the ice cross gem. I don't remember for sure if I got it before the portal or while I was exploring inside. Either way, it wasn't hard to find.

Now, when the battle starts you'll want to stand to the left of center about even with his knee as close to him as you can get.
Aim towards him and spam the 'X' button to shoot ice crosses at him.
If he starts to swing his whip, just move to the right and stand in front of him still spamming crosses. After the rocks fall, move back to the left and spam away.
Stay front left and his fire can't hit you. When he calls minions just stay left and kill them with your normal attack. I found the ice glove to be quite effective for this.

That's all there is to it. He'll rotate through his attacks a couple times and it'll be over. You most likely won't even get a scratch.

Be sure to pack a few Mana potions along as well.
Good luck🙂