I duelclassed a kensai to a mage and it say that the class is inactive. My lvl are 10/14 and he really sucks. Why does everyone think that a kensai/mage is good. He can't do anything. Low thaco and very high ac i think it was 5 with wuth Robe of Vecna.

When a class is inactive, the character will no longer recive experience in that class, its all in the instructions for dual classing, you will no longer get XP as a Kensai, nor will you get the advantages a Kensai gets when s/he levels up, you will only be able to better yourself as a mage.

Btw, i did it by playing as a normal mage, and just fighting when I had to.

I don't know what you did wrong, but my Kensai Mage is great. He fights all the time along with my main fighter. He has an AC -5, 130+ HP, casting time -5, immune to level drain, permanent vocalize, boots of speed, cloak of mirrors, dualing with Dakkons Zerth Blade and Celestial Fury. He plows through humans and demi humans(and anything that can be stunned), mages, and vampires. And he can hold his own against anything else. His contingency has kicked in only a few times(hp 50%), and I rarely need to use healing potions with the Gaxx rings. If I can make a suggestion, redo your character with shadow keeper. Dual him at level 9. Equip him with everything above, as well as the ion stone +10% hp, find a familiar, level up max hp, and you should get a great character. And my character has good thaco. Also you have to have 18 dex. Sorry, it's a must.