Use another character to absorb attacks (aka. a tank) and use the Kensai as a peripheral warrior.
As you stated, the Kensai armour class is relatively poor and the class is very vulnerable to direct melee attack.
You could possibly use boots of avoidance to reduce his AC versus ranged attackers, possibly the bracers that increase dexterity to 18 (depends on the characters current dexterity...) or malakar as a dual wielded Katana (reduced AC versus slashing weapons by 2...).
Good luck
