General

i went
1 venom, 1 smokescreen, Max Meso Mastery, Max Bstep, 9 mw, max assassinate, and working on Shadower Shifter right now, then after shifter mw or taunt.

On my Shadower, I'm doing :

1 Assassinate
1 Shadow Shifter
1 Smoke Screen
Max B. Step
Max Meso Mastery
Max Shadow Shifter
Max Maple Hero
Max Assassinate
Max Smoke Screen
Max Taunt, maybe...

After 10 SS I would recommend 9 mw instead. Taunt has a slow animation and won't work at LHC.
After that you can either max taunt/mw/nate/ss. Personally I'm going to max SS, but it's up to you.

You could go for maxed shifter. That's what I'm gonna do

I'm doing Shadow Shifter up to Level 10 and then Taunt.
But you can also do Shadow Shifter to 10 and then start maxing Maple Warrior or Assassinate, both really expensive.