As to how I did it in the end...read on if you are interested......
Firstly, I took all henchies except the mage. The mage is a liability as he uses meteor storm. This causes the enemy to scatter which you do not want! You need to keep stuff attacking in one place in this mission.
Well, getting to the keep was easy. Once in the keep:
A lot of the problems stem from the fact that the idiot dwarf king you are helping seems to have a suicidal streak. He will not attack the thing you are attacking. Instead, he'll just rush headlong into the enemy and keep going waaay ahead of where your group might be fighting. He seems to like going one-on-one rather than staying with your party. So you have to manage the king in order to stop him rushing off too much and getting into trouble:
1) Once at the keep, take out the groups of baddies around the edge of the arena first being careful not to agro the 2 groups of monsters and the Boss baddy on the arena floor. This is possible if you keep near the outside edges of walkway around the arena where the balista are. You do not want to agro the big groups on the arena floor as the king will head off down there so you have to follow and protect him. Not too bad but what you are trying to avoid at this point is killing the main boss and triggering the cut scene.
2) So, once you have killed the monsters around the edge of the arena, ignore the ones on the arena floor for a bit. Instead, follow the walkway around to the far side and use the balista to kill the large group of monsters waiting outside the wall This will give your allies more of a chance once things kick off.
3) Now go and kill the boss on the arena floor. This triggers the cut scene and the main attack begins with you standing up on the platform with the king.
From now on, I'll refer to left and right of the arena. This is from the perspective of looking into the arena from the platform where the king is.
4) As you have killed that first group of baddies on the right with the balista before, you have a bit of time. In general, you have a bit of time to use the balistas as the first wave or two of enemies turn up. I think the next lot came from the left but they don't stop at the balista explosion points so you can't do much about them. Leave the undead allies near the left gate to deal with them. Over on the right another group should turn up which you can take out with the balista. Think there is the first of the Bosses with that ( a dwarf). In general, you can take out all but the boss with the balista.
5) I then ran down and helped the undead to kill the dwarf boss at the right door quickly. You should have time to do this as another group comes in from the left door. Keep an eye on the map though MAKE SURE YOU GET BACK UP TO THE KING ON HIS PLATFORM before this new group get there though...even if it means leaving that dwarf boss still alive.
6) From now on you have to defend the king. Stay very close to him. Importantly....always try and get right to the back of the platform as the enemy group approaches so that enemies coming up the stairs to the platform out of the arena have to come as far as possible. This will help not to agro too much stuff at once as new enemy enter the arena and to keep your henchy healers from running into combat and getting killed!
7) The next group or two has a couple of Jade bows and some White Mantle bods....they are not too difficult as long as you stay close to the king and don't let him get caught alone.
8) The hardest bit in my view is when the Mursat Boss turns up with some Jade bows/warriors, other mursat and some white mantle all at once. This is where I failed every time before. The Mursat boss is as tough as granite! You also have some enemy healing bods in the mix. My mistake was to take on the Mursat boss too soon as he was usually at the front. Don't! He will take all the punishment you can deal for a loooong time which just allows more baddies to turn up! Take out as many of the normal Jade's, normal mursat and the White Mantle first. It gets a bit bloody (or it did for me) and you may lose a hench or two but as long as you don't lose the healers you are OK. Finally, take out the Mursat boss.... a lot easier now without everything else attacking you too!
9) The rest is plain sailing really. You get a few more attacks with a couple of Jades and the odd Mursat but nothing like as bad as before. Just stay around the king and defend him.
10) The main White Mantle bod will come in after a while with a few other white mantle in support. He is a whimp compared to the Mursat Boss! Just take out the other white mantle supporting him then blitz him.
Job done
