I am a ranger/warrior lvl 20. I use a sword rather than a bow for the dps.
The skills I use are :
Distracting Blow
Apply poison
Sever artery
Dust trap
Barbed trap
Whriling defense
Healing spring (the only way to avoid the king to die)
Capture Signet
I have :
Expertise 14 (10 + 3 + 1)
Wilderness Survival 13 (10 + 3)
Swordsmanship 11
My HP are : 380
My mana is : 31
The henchmen I group with :
2 clerics
2 tanks
1 necromancer
1 ranger
1 elementalist
Me
The king
I had quite a hard time with this mission but my aim is to finish the game with henchmen (as I can't do it completely solo). And I finally did this mission. Getting to the king is easy. The first difficulty I encountered was when I had to clear the 2 groups of arcanist near the boss in the fort. It's a bit a matter of luck, but the point is to always stick to the target the king is on. Even if you have to switch, do it. If you tank, the king won't so will not take any dmg except for the hexes the mesmers casts at first on him. I usually throw one healing spring on the pull when I'm right near the king so he doesn't die on the pull. But last time I didn't even need it because the king will normally go to the target right behind the nearest one, on your right. So you can predict what target he's going on.
After the door open, the only difficult time is when the mursaat boss comes. All the rest is nuts. I stay near the king and I try to stick to back of the fort. I let the mobs come the closer of the king so I don't have to run from a place to another avoiding the risk to not see a mob that would sneak. When the mursaat boss come, you have to get rid of every single mob that comes with him especially the monk mursaat. then I stick on the boss. But if another mob comes I switch targets even the boss is low life (generally he's a necro and he can drainlife healing himself). What kills you and the entire group is adds. At a time the boss will stay alone and then he's dead (distracting blow helps a lot, or any interupt in fact).
After this mursaat and jade mobs will come 2 by 2 rather slowly. Keep cool, let them come, kill them don't rush them. When Confessor Dorian comes with his mantle-boys you're about to win in the easiest way. I got down to the door to catch them. But it's really a fake fight.
Ring of fire is really tricky, I got whacked by 2 jade bows and a seal and that's only 3 mobs. But everything is doable

I hope this will help you a bit.
PS : a little hint I use, when caster or ranger mobs I rush them so I tank first but when warrior mobs I pull with agro-range (white circle on the map) then I backoff, henchmen won't rush if they don't get hit.

They either learn to fit in or drag the rest of the team down with them.