You only need to look after the king on the way to the keep as he has a tendancy to run into the fray and get all kinds of icky crap on him like poison and lord knows what else.
A */Mo with healing breeze at +7 regen and Vigorous Spirit will be able to keep him upright with no trouble at all. Thisis how I approached it with a full henchie team:
Once at the keep, as son as the cut-scene finishes man the right hand catapult, kill or weaken the mob incoming - pick off anything that makes it through from the safety of the walkway over the entrance.
Kill the second lot on the same side using the catapults.
Run to the other side and help the undead on the left side, then fall back to the steps. (if you have the people to do it, keep one person on each cat at this point).
Fend off the giant herders and enslaved giants from the steps, I usually take out the giant herders first as i find them the most annoying.
Now you should be upto the first waves of Mantle and Jades. These will come in dribs and drabs of two, if you simply wait at the top of the stairs these are stupidly easy to take out, you should have at least 2-3 seconds between assaults from pairs. Resist the temptation to go down the steps and meet stuff.
Eventually a random lvl28 boss type will turn up. This is the part that always caused me the most trouble. If you're really unlucky you'll get a monk boss (feckhead the Undying i think he's called) I found this guy impossible to take down on this mission with henchies. Depending on your group this is much easier if you get a Ele, Nec or Wa boss.
For me the best approach was to take out the guys that he brought with him first (mantle / mursaat / jades) and then concentrate on the boss. You'll likely lose one or two team members at this point, don't panic, keep cool and plug away - once you kill the boss you will get time to ressurect. The only time I'd say you should erz while in this fight is if a healer goes down or your at 3 men - at this point a rez signet might be wise

otherwise you're diverting fire away from a bad guy and endangering the group.
After this its all plain sailing of mantle and muursaat / jade in groups of 2-3. just stay with the king as after this point he tends to wander down the two stairways.
oh and don't e scared by the size of th final group - let them into the arean and they'll fan out and you can kill them one at a time while sitting back and smoking a hamlet or seomthing - they are pansies!