What is your primary profession? But anyway, here is what I used:
Minion Master hero.
Rit lord hero.
Koss as a sword warrior w/expel hexes (I hate faintheartedness with a passion).
Then used... Klim, Mhenlo, Herta, and Sogolon. Myself as a Dragon Slash warrior with Shield's Up! and Watch Yourself! for some passive defense. And I don't use LB gaze.
I foolishly played a few of the HM missions with my Lightbringer title off so I can only assume it would've been even easier with it on.

As far as the mission is concerned, drop the warlocks first so they aren't taking bodies and spreading hexes around. The clerics don't do much healing at all, so I rarely take care of them first. Never engage a stationary group unless you just finished off one of the groups that come automatically. Just play it safe and slow.
For the bonus, I took out everything but the final harbringer before trying it. After killing the automatic group, I would flag everything in the middle between the two spirits (the ones that appear when you kill the harbringers). I would then run off up the stairs to aggro the bonus group (then back to the bottom of the stairs and slightly to the side so I could see when another automatic group would come), and before they would engage the spirit, cancel the flag and wipe what came down. Before continuing to drop the light of sehborin (sp?) I would rush back to aggro the next group that spawns automatically so they wouldn't kill any of my spirits. I hope that makes sense.