I completed this quest line with my 52 Troubadour. The assassin is a level 53 (one-up tick), and I was able to kill him without my mercenary or pet. I know that it says you must be in stealth mode to see him, however I stumbled upon him without being in stealth; he was on the chains above the castle guards. I did however go into stealth mode, only because it was listed in the steps, and when I approached the mob, he was suddenly surrounded by a purple haze and he became invisible. Despite him being invis, I was able to target and knock him down with my ranged, and fight him on the steps. Just FYI, the guards did not help me fight this guy, LOL. I am sure that I put on a good show for him, though. This entire quest line was fun. I took the time to read the dialog and really got into the storyline. There was a small twist I didn't see coming, and one twist that I felt could have been added. I hope this helps a bit.