Lightwolf: Well, I didn't know that von ran Disguise that way.
Agape: It's more a problem of Disguise not doing what it's supposed to do.
You can't do observe for Assassinate, do a ranged Assassinate, or Sneak Attack outside of 30 feet. This isn't even something I invented either, Sneak Attacks don't work further than 30 feet in Pathfinder.
Assassinate also only works if you surprise your opponent.