Darknights

So basically, before I fully unlock the Isle of thunder I'm using the Break down the wall 1man scenario to max out my Black prince rep by grinding the unlimited respawn mobs in the Scenario. I've set up a path around the area I want, got a fairly good combat profile. But for some reason the bot will only attack enemies that aren't already in combat, which is frustrating because it would kill so much quicker if the bot would assist the NPC's fighting the trolls. I have the 'Can attack units already in a fight' thing ticked, so I don't know what else I can do to fix this. Any help please? :) edit: Okay well it's just going for the nearest one, and because they insta-spawn thats usually one at full health, is there a way I can make it focus on the ones who are already in combat?