Well, this might be too late for the OP, but maybe not for others. I did this mission last night, with the following hero setup:
Dunkoro as a HB healer with [Extinguish], [Spotless Soul] and [Spotless mind] along with [Words of Comfort] [Ethereal Light] [Dwayna's Kiss] and [Healing Ribbon]. He easily kept all four of us alive, although only Melonni really fought.
Melonni with [Avatar of Balthazar] [Heart of Holy Flame] and a bunch of attacks. She only had 7 skills because I got in an argument in guild chat and forgot to add one more skill. She did perfectly, though. Pretty much anything went down really quickly.
Gwen with [signet of judgment] a bunch of other signets and [holy wrath] which I cast on all four of us.
Myself as a [Broad head arrow] ranger/dervish. I took along [heart of holy flame] to give myself holy damage, as well as [dodge] and [zojun's haste] along with [troll unguent]. Had a couple of attacks, and of course [distracting shot]
Once you get to the fortress, flag the Monk and Smiter behind, with Melonni just about outside their aggro range. Go ahead and pull the undead that you need to kill. Pull them to Melonni, and either tank with her (if not a ranged/caster) or move a bit back once they focus on Melonni. Make sure they're in range of the smiter, and that you don't go too far away from the monk.
It shouldn't be a problem, since they go down REALLY fast. I did 200+ damage with [heart of holy flame] + [sloth hunter's shot].
After the first mob of undead is gone, unflag and run, get the tablet, and run ahead. Kormir won't attack the ghost if you're moving, and the heroes will follow you (or you can set them to Avoid combat).
Drop the tablet next to the pedestal (not ON it) and repeat the procedure on the other side, for the next tablet. I used my run skills to just run and get the tablet and ran back, followed by the undead, who then died

. It's possible to lure them, but I didn't want to bother flagging my heroes outside the fire traps.
Once you've got both tablets next to the pedestals, pick them up quickly and put them on the pedestals. The sunspear ghosts won't have time to run there. But hurry and pass through the door anyway.
If you're a squishy, another easy option is to wait until the Sunspear ghost is out of range, and flag Melonni ahead so she draws the aggro. Then you can either flag her backwards so she brings the enemies close to you, or you go forward to help her. Or bring a bonder instead of the Smiter, and have them bond Melonni. Then give Melonni a /Mo build and micro her skills.
Make sure you either bring lots of spammable partywide condition-removal and healing, or give her [Avatar of Melandru] so she'll be immune to conditions. I haven't tested this, but if you cast [Heart of Holy Flame] after [Avatar Of Melandru] she should be dealing Holy damage instead of earth damage. Doesn't matter as long as the bonder keeps [Holy Wrath] and [retribution] on her, though. Those kill the undead pretty fast.
A 55 monk can do this pretty easily, I think. At least until the final boss, that is. And you can't do it yourself, since Kormir will follow you. Maybe get a friend to help?