I believe you've got to beat the Tower in order to unlock him.

Thats 7 floors of pure torture but wait until you unlock the Catacombs.

The worse is yet to come.

Tips I can give you on how to beat these dungeons:

1: Learn how to use your class. For example learn the width of the attacks for melee classes and learn the mana consumption of the mage class.

2: Learn how different enemies fight. Each enemy has their unique fighting pattern which if you know beforehand you can prevent yourself from taking damage. Also, PRIORITIZE ENEMIES IN EACH ROOM. Take out the enemy which is of most threat to you, that'll make things way easier.

3: Dodge. Pretty self explanatory.

4: Don't be stingy about using your spells or buffs. If you're in a tight situation, USE THEM DON'T SAVE THEM.

5: Know the upgrades to the classes. And I'm not just talking about the buyable upgrades from the menu, I'm talking about Emberforge upgrades. Always aim to get gear that improves your survivability and crowd-control. Hell, its pretty much Warframe logic.

6: Hats. Find yourself a nice hat and make yourself feel like a badass.