Okay, after 25+ attempts, I finally managed to get through this quest. Here’s my step-by-step – YMMV.
Summary: The mission is to escort Trahaerne to the tombs. You will get in lots of fights, which were generally very hard (three main points where you may die), and if things don’t pan out well then you can experience a number of bugged points.
Steps:-
1) Talk to Trahaerne (T from now on) to kick off the story.
2) He will start zig-zagging down the hill. Follow him about 1-2 persons length away. Let him initiate combat where possible. At the bottom of the hill there's a mob on the left which he bypasses - once he is level with it, you can initiate combat - I'd recommend you do so. There's then another bad-guy in the gulley when you turn right, again let him initiate combat.
2-bug) If T gets stuck during 2 any time, try walking up hill 5-10 lengths away, then run down hill and through T. This tends to unstick him
3) Mob fight 1: When you have killed the mob in the gulley, T will run forward and trigger ~10 mobs around you. Therefore, as soon as this mob is killed, I'd recommend laying traps etc around the gulley (remaining behind T). Your overall aim here is to keep the non-named NPCs and yourself alive - not an easy task. It's not uncommon to die here. If you do, I've been able to respawn and carry on from here - T gets stuck later but I don't think it's related.
4) Following this fight, T will walk out of the gulley, and turn right into the valley. If you stay far enough back, he will pause just after leaving the gulley, allowing you time for your elite skill timeout to expire.You _will_ need everything in your arsenal for the next bit, or at least I did.
5) When you turn into the valley, you’ll find a number of mobs. T will run right up to them before commencing an attack. Mob fight 2: You can't kill the weaker mobs, die, respawn, and repeat - if you don't kill the risen abomination then they respawn also. I recommend targetting the risen abomination (big b*stard) right from the start. Really easy to die here, especially if all your non-named NPCs died in fight 1.
5-bug) This is the most common area for cataclysmic bugs. If it occurs, you’ve no choice but to log off and restart the entire mission from scratch – this can lead to losing a lot of money due to having to get your kit repaired – you have been warned. I found the following tricks worked. a) If T is down, but a couple of mobs are still fighting, if you rez T then he will join in and then not bug. b) If T is down, but you’ve killed all the enemy, then as long as other named NPCs are down, you can rez T first. He will rez the others (help if you want), and then continue.
6) After the fight,T will tell the others to remain outside and guard the tombs, and will go inside. Follow him. Again, stay behind T and let him initiate combat.Unfortunately, he's a complete imbecile and so walks into every ambush going. This can lead to you dying. If you need to respawn, from my experience, you're screwed.
6-bug) I experienced the same bug a couple of times here, and am not 100% on causes. On the stairs inside the tomb on one run, I had a couple of NPCs with me – not sure if that was the cause. I also tended to run ahead through the ambushes, pushing back mobs and using AoE. Whilst that kept me alive, T got stuck. The lesson – stay behind T and let him initiate combat.
7) After the exitting the stairs, I didn’t experience any other sticking bugs.The end section itself is a bit weird, full of non-sequitors (the rite fails, you say it seemed to work for a while, but as a player I've seen no such evidence). Also you're told to chat with T but there's still mobs transporting in, including one mob he's still recommending you chat to - very weird.
8) You’re done. And good luck on the next mission, as I’ve heard (haven’t played it yet) it can also be buggy…
TL;DR: Walk behind Trahaerne, let him initiate combat, rez before combat ends if possible, rez him first if multiple named NPCs die during combat.