Actually, the mount is the only tank you need, even for unfair.
Horse can be enought, although other choices are definitely better. The most I could get up to 75 ac outside combat with elk.
I don't count bonuses like fighting defensively or crane styles that work only in combat.
Due to the fact that I play TT, AC should be even higher on the base game.
Elk AC breakdown:
AC Bonus Breakdown:
-- Touch AC bonuses --
+10: Basic AC bonus
+14: Dexterity bonus
+4: Deflection bonus [Angelic Aura]
+1: Dodge bonus [Dodge feat]
+1: Dodge bonus [Haste spell]
+4: Dodge bonus [Fighting Defensively + Crane Style]
+3: Morale bonus [Helmet of Comradery]
+2: Insight bonus [Foresight spell]
+1: Untyped bonus [Ward Master's Amulet]
-1: Size penalty
-- Non-Touch AC bonuses --
+24: Natural Armor [Animal Companion bonus]
+6: Natural Armor [Frightful Aspect]
+6: Natural Armor [Inspired Rage]
+5: Enhancement to Natural Armor [Amulet of Deep Roots]
+8: Armor bonus [Bracers of Armor +8]
+4: Shield bonus [Shield spell]
Total Armor Class: 92
Touch Armor Class: 39
Conditional: +4 vs melee
Not using the best tank items, gave those to my boar