Barkskin = base 16 AC.
Bracers of Defense = +2 AC
Shield of Faith = +2 AC
Mage Armor = +3 AC
18 DEX = +4 AC
Ring of Protection = +1 AC
Stand next to a level 6 battle cleric with Herald of Battle = +1 AC
Have Shield and Counterspell slotted.
The result is a base 29 AC mage with the option to Shield up to 34 AC (yes, I have done this). Arrows and melee attacks are almost useless. Single target or area spells that affect the mage can be Counterspelled. Note that Barkskin and Mage Armor have long enough duration to be cast ahead of battle and can be cast from scrolls (Mage Armor scroll is a common recipe, too) and Shield of Faith is a bonus action so the first Paladin or Cleric who goes can cast it without using up an action.