As for the Deathknights, the disguise does not and should not work against them. There are two options I know of for dealing with them that do not involve cheating.
You can level up in the Battlegrounds, and gain gear there, to where you are powerful enough to slay them. Downside: Many people find leveling in the battlegrounds is boring and makes the game too easy for them.
You can sneak past them by being careful and learning their patrol routes. Downside: You don't get any XP/Loot from them if you just sneak past (unless you go back and kill them later), and you don't get the satisfaction of beating opponents your companion told you are far too tough to handle.
