To be honest I didn't find the last boss that hard in comparison to the Cube Boss.

Spoiler Alert, click show to read: 
After the Soul Cube gives you the hint when you get it that "only it can defeat Hell's greatest warrior", not to mention all those tablets it didn't take much to figure out that only the soul cube could harm it.

I just ran around in circles around the hellgate wasting the Imps and blasting the boss with the Soulcube when it was charged with the standard Five souls