
First rule... Just forget humans. Unless you know the people in your group (such as if our guild does it), just don't bother. It just takes one to do something slightly wrong in some places, and the mission is over. It also takes one disconnect, and you may as well just warp back to the outpost. Henchmen, on the other hand, are more intelligent than some humans (and won't disconnect).

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First Part:
Ok, for the most part, the whole first half is pretty simple. It can be done with humans and henchies, and as long as something really unlucky doesn't happen, a balanced group can handle pretty much everything. I'm not really going to talk much about this, except, watch the movement of the red dots. There are quite a few wandering groups, just see what their cycle is and attack accordingly, trying to pull one group at a time.
Also, another thing: If you go with humans, you're not there for farming, and you wish to beat the mission... If the group has major issues with the first part, take charge and get the group in line right away. The last part is potentially insane in comparison and a disfunctional group will never make it.
Also, don't let Sir Shorty follow a warrior or ranger. Keep him with a spellcaster for protection away from the heat of things. If he dies the mission is over.
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Last Part:
This is the tough part. You'll get to what is obviously the end of the road, and there is a walled structure with a couple closed doors and some mounted ranged weapons. Try to go up and defeat the mobs on the wall in single groups. The ideal situation, if possible, is to defeat all of the mobs on the wall before dealing with those surrounding the boss.
When that is done, wait. Check the area beyond the fort structure. You should see some red dots hanging out by at least one crater. Figure out which weapon is aiming at it and fire it over and over until all applicable red dots turn gray. They won't run at you if you do this before taking the boss.
Then go around and take the groups surrounding the boss, and then the boss. Be sure that the boss is your last target.
Then, expect to be on the edge of your seat for at least 10-15 minutes. The keep is about to be stormed by mursaat and white mantle mobs. There are a few things you can do...
With Humans:
I'd recommend that you get two non-monk spellcasters at the artillery for a while. Tell them to stay there unless called by whoever is the leader of the group, and get them to fire at the mobs as they come. They can significantly damage (or even kill) them so everyone else has an easier time handling them.
If the group has two tanks and two monks, you can choose split the groups to keep the mobs at both gates, with one or two floaters in the middle. This is effective but can easily break down, don't try it unless your group is competent enough for it.
I'd personally recommend that the group stays at the middle as one, and if things get bad, run up and protect the king. If possible, prevent them from going far enough as to aggro the king. Once this happens it's bad news, because the king has now made himself more vulnerable by walking away from his little platform.
If you have a ranger, get him/her to take barbed trap (or another trap, that's the one I'd recommend though). The ranger should keep traps at both doors at all times, and if the need or opportunity arises, trap the necessary stairs. This will make the battle somewhat easier since the mobs take a rather predictable path through the keep.
Keep going, target bosses after the lesser guys, and take care of white mantle first. I prefer white mantle, to healers, to elementalists, to necromancers, to tanks, to rangers (the rangers have an ability to block the majority of what comes their way). I don't remember if there are mesmers or not. Eventually you'll get a message to talk to the king. Do just that and you're on your way to the Ring of Fire.

With Henchmen:
Keep the above in mind, and try to go with the center strategy. Try not to get the group to run too close to a door, because it will open up the way for the other side to get to the king. Stay at the center, pull if you can, and if you can set traps do that when possible. Don't worry about the artillery unless you get a weird gap in mobs, and if you feel adventurous take a peek on one side to see if you can drop one or two projectiles on them.
Can't say much more than that, just improvise, and don't lose faith in your loyal AI team.

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Lastly, expect to do this more than once. It'll be painful, but it will eventually be done. There is a certain element of luck with this, for the mursaat seem to come in at slightly different rates each time. Also, there is a healer boss that will come in about half-way through the attack. If you can get through that wave, the rest is downhill from there.
And lastly, do not do the bonus if you're trying it to merely get to the Ring of Fire. It will make things really difficult.

Anyway, good luck, and feel free to ask me to come along if you're about to attempt this.
