So.. I've been testing out the shield and it apparently gives you a percentage to your BASE defense. So for example if your base defense is 3000 and you add 10% to all spell damage ( 10% to poison and 10% to fire ) you get 300 extra defense so you have 3300 defense. Also, as far as I can see, adding TCD% does not help, meaning that if you have 3000 BASE defense then add 10% to all spell damage ( 10% to poison and 10% to fire ) and then add A SHIT TON of TCD%, the shield will still add only 300 extra defense. So basically, to optimize the bonus from this shield you need to get gear with high BASE defense and add Enhanced Defense.
Also, I started struggling a bit with sin at lvl 108. Mainly because of mana issues against bosses. I have 300 mana on attack, 5000% to attack rating, invested all in to dexterity, but basically as soon as I miss a few hits my mana is gone and I loose twin fang strike.
So if anyone has Metalhead SU on single player and could send it to me, I would really appreciate it.