There is one item that I believe is essential to the Kensai: The shield amulet. This casts the level 1 mage spell shield on the user for five turns. It sets base AC to 4, and + 2 AC vs. missle. With this active, Kensai AC bonus, Ring of Gaxx, Cloak of Protection + 2, Ring of Earth control, and Ioun Stone you can get down to -8 AC.
The amulet can be found relatively easy from the Fighter's stronghold quest. It has around 50 charges, and can be sold/re-bought cheaply to recharge it.
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"This place will be your tomb."