Use the shield amulet all the time; with Kensai AC bonus, Dex bonus, and other magic items, you can get a Kensai down to -6. You'll find a couple more items that will lower your AC a few points.
Oh, you can sell the amulet and buy it again, and it will be recharged too; don't worry about using up charges.
Deathspawn: Kensai can't wear armor.
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"This place will be your tomb."