I've read alot of opinions about when to dual a kensai to mage or thief, according to the game the kensai gains these level dependant bonuses: +1 to hit/damage every 3 levels, -1 to speed factor every 4 levels and 'kai' ability every 4 levels. This seems to me that the best level is 12 because you become stronger in all of these abilities at that level, so why do some claim that 13 is best level? What do you get there?

One extra 1/2 attack. And extra attacks always make a big difference.

Dual to a mage at level 21 :D (22)

Dual to a thief at level 24 :D (28)

Plus an improvement in Saving Throws.

Dualling at level 21 is not recommended, unless you are willing to sacrifice A LOT of spell slots. The difference in melee skill between level 13 and 21 is neglible, and HP is NEVER an issue with high-level Mages.

Using the infinite spells strategy, losing out on a few high level spells isn't really a problem.

A level 13 Kensai has a THACO of 8 and a +3 hit/damage bonus. He has max 9d10+12hp. His saving throws are higher. 8 weapon proficiency slots (5* flails OR katana, 3* dual weapon proficiencies).

A level 21 Kensai has a THACO of 0 and a +7 hit/damage bonus. He has max 9d10+36hp. His saving throws are lower. 11 weapon proficiency slots (5* flails AND/OR katana, possibly quarterstaffs. Dual weapon style is recommended).

The point of powergaming a Kensai/Mage is to make the character as POWERFUL as possible in the LONG RUN. The L21 character is able to use magic to boost his fighting abilities massively (improved haste). It is entirely possible to have the Kensai/Mage with circa 250 HP/500 HP when boosted.

I will concede that it is a pain in the bum waiting for the Kensai abilities to reactivate. However, there are so many experience points available in SoA for a soloist, you are able to easily hit maximum levels before moving onto ToB.