The combined armor rating of helm, torso, gloves, and boots must be at least 2000. Items with rune slots can be very helpful by placing 10% and, at level 25, 20% extra armor runes on the items. The restricted alignment dungeons have a special chest at the end that has a good chance of holding armor with the 20% rune already, even if you aren't level 25. Every 5th ascension emblem, starting with the second, from the dark god gives an extra 5% armor to a max of 15% at level 40. Other than that, just upgrade your best armor with the runes until you have 2000 armor. The lightning shield spell will give you an extra 25% temporarily, so you would only need 1600 before casting it. Be sure to go to the inventory menu and switch equipment for it to pop!