I love when you transform him into that big demon/beast,... Is there a build I can use around that beast? and what type of gear I should be going for... using fencing as my weapon.

I have only seen one player using that form, as an archer. Check here for most of the benefits of the form. I THINK that beast also is immune to poison (up to lvl 4?).

Since you cannot cast in that form, you are kind of limited as to which support spells you can use in battle, like divine fury, enemy of one, cure posion, etc.