The hardest part about the final fight is taking down the boss' spell protections. If you can do that, the boss is vulnerable to sustained physical attack (Greater Whirlwind). If you have the flail that slows enemies down (and if you're a Cleric / Ranger, you should - it's arguably that class' ideal weapon), that works like a charm. The elemental damage goes through Stoneskin and if you found the two extra heads, the chance of the slow effect goes up to 50% without a saving throw!
Of course, if you have Ascension, it becomes a little harder. Even if you do have Ascension, there are still many different ways to finish it. One of my favorite accounts is
this one.