The arcane warrior is surrounded by a shimmering shield of energy that blocks most damage and grants large bonuses to armor and all resistances. When active, however, the Shimmering Shield consumes mana rapidly.
It is possible to completely offset Shimmering Shield's -10 penalty to Combat Mana Regeneration if you choose the correct equipment and skills. Combine any of Wade's Superior Dragon Armor Sets (+2 regen) and Andruil's Blessing (+2 regen) with Cailan's Arms (+5.75 regen) and the second tier Combat Training (+0.5 regen). The previous numbers are from the PC version, on Xbox and PS3, you have more equipment options, as the regen amounts are twice as high (except the +5 set bonus from Cailan's Arms).