How viable is a human Kensai that has the 2-handed sword +4 and 2-handed weapon style +2 skills?

He also has 18/98 Strength, 18 Dexterity, 18 Constitution, Intelligence 8, Wisdom 3, and Charisma 18. (Yeah, I know I had to take a hit in the brain to get the others so high... didn't feel like rolling virtual dice all day...) I am new to this game (and this type of game) Any pointers on how I could have made a human kensai better are most welcome!
(Btw I don't dual wield... I just don't like doing so.)

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...Plus, by Throne of Bhaal nearly everything starts to be able to hit you practically regardless of armor class, so hit point count becomes more important for tanking anyhow.
Just to give you an idea: on Insane difficulty with all damages taken doubled, Rasaad the monk NPC (he has perhaps similar AC and high innate Magic Resistance) can be erased from the game by enemy fighters in 1 second. It doesn’t matter what he can do, if he can’t survive.
To sum up: viable or not, is primarily a matter of your difficulty settings and play style. All classes or kits can be or can be made viable.
On the offensive side, viability refers to either damage output or the ability to kill without direct damage or both. A Fighter/ Mage is superior. They all wear no armor, but a F/M not only can take a lot of beating (similar to Kensai->Mage) but can also initiate 10 auto hits per round with full damage plus all possible effects that come with physical attacks, using Time Stop. A Kensai->Mage is also superior, though they focus more on magic for the kills.