Make sure that you save at least twice at the beginning of the game (that and make sure that autosaving settings is on to) so once you have done that, enter the door where you first learn how to sneak.

Now search for boxes and look for a paintbrush. Once you found one, make your way to the part, where Glenroy says "I don't like this let me take a look", as soon as he does that make a stair like thing and you be able to get up to the part on the left side where the Assassin pops put out from the door on the left side.

As soon as you have done that, stay on top of that area and use the marksmen skill and destruction skill on the other assassin across from you and then attack the other assassin with your Blade or Blunt, it does not matter and this also helps with Blocking also.

Make sure you have plenty of potions.