Go with a guisearme. Get a high dex, good strength, at least a 13 intelligence. Pick up Combat Reflexes, Combat Expertise, and Improved Trip.
Person moves through a square you threaten - you take an AoO and attempt to trip him. If you succeed, he's prone (-4 to melee AC) and you get a free attack. If he attempts to stand up, he provokes an AoO. Rinse and repeat.
As far as getting to be a Kensai by level 8:
You already have Combat Expertise. You need Weapon Focus, which can be easily gained by a level of fighter (I'd suggest 4 levels of fighter, though more is acceptable).
You need 5 ranks each in Concentration, Ride, and Diplomacy. Ride, obviously, is easy. Concentration and Diplomacy are cross-class skills for a Fighter. However, they are class skills for a Paladin.
My suggestion might be a Dwarven Fighter 4/Paladin 2. (You're actually eligible at Fighter 4/Paladin 1 or Fighter 3/Paladin 2, but Weapon Spec and Divine Grace are both really handy, so...) In addition to Combat Reflexes, Combat Expertise, and Improved Trip, add Short Haft, Weapon Focus (Guisearme) and Weapon Specialization (Guisearme). Bump your Charisma up as high as you can get it, for the bonus to saves. If you're willing to give up being a Dwarf (human is likely your next best bet, though an elf gets you +2 Dex), you can avoid the Charisma penalty, but it's entirely your call. Also notice that one of a dwarf's benefits, the ability to move full speed in heavy or medium armor, is likely to be negated by your high dex build. Note that if you did go with an Elf, you may want to go either Fighter 4/Paladin 3, or Paladin 3/Fighter 2, to avoid the multiclassing penalty.