After three attempts I managed to get things done the fourth time round. I lost quite a lot of repair silver, but hey. Unless I’m very much mistaken, I did not have access to the cannon, only the trebuchet on Trahearne’s right (human ranger, Order of Whispers). The acolytes walk in range of the trebuchet a few times, but I was a little too slow to spot them at first. In the end, I managed by selecting an acolyte and keeping track of its path across the courtyard. While it was out of reach, I killed everything and anything in range of the trebuchet. When the acolyte walked in range, I killed it. You have to be quick, though, and start targetting them immediately. By the time the two acolytes were dead, every other mob had died save for a few more brutes. I was left alone on the trebuchet, no mobs came up to kill me. If you’re patient, this should do the trick, I think.