Despite what I and many other people say, TI is still the best way to level and will get you used to functioning in a party setting, relying on others. The sole problem with TIs is that sticking only to that form of leveling won't prepare you for the end-game, and it certainly won't do anything for your performance in WoE. Everything in moderation.
40 DEX in that build should be fine for leveling as an Assassin, even if it is the bare minimum. You can dump your excess points into INT (18 points or so) if you have trouble maintaining SB, otherwise using the remainder to boost DEX to 47 would be a decent idea, which will boost your HIT by a respectable amount and your status attack marginally.
With 40 LUK and the full Morri set you still won't have "perfect" CRIT even when paired with a Katar, so you may also want to invest in Soldier Skeleton cards or Racial Crits for your weapon.