Personally I'd dual to a thief at from the Kensai at level 24/25.
L24/L28 Kensai/Thief. +8 hit/damage.
L25/L27 Kensai/Thief. +8 hit/damage.
I know it takes a while to level up back to the thief, but it's not as if you don't get much experience in the game. On the plus side, you will have uber fighting/badguy wonking potential when you do finally get to L25 as a thief (not too hard to do). Then we want the use any item ability and we laugh. Hahaha!
