Through my experience playing Guild Wars, I have learned the following through repeated attempts at conquering the Thirsty River mission. Had I attempted the mission with trusted guild-mates, I would not have learned these valuable lessons.
Lesson the First: Strong plus Stout equals Stupid. Unless you know the player behind the Warrior/Monk personally, or have observed their team skills in action, consider them a grave threat to the safety of your party. This is doubly true in the Thirsty River mission. Do not be surprised if said warrior makes a beeline to the enemy priest, leaving the rest of the party to get perforated by Forgotten Avengers, chopped up into bite-size pieces by Forgotten Champions, and fed to the Forgotten Cursebearers, in a lovely white wine sauce with couscous and roasted vegetables.
Lesson the Second: Conformity is its own reward. A Necromancer who does not use an Approved Build is not worth having in your party. Approved Builds include the Minion Master, Spiteful Spirit, and the battery. These builds have absolutely no flaws in them whatsoever, and their utter perfection is well-documented. When I passed Thirsty River with my Necromancer/Mesmer, I used a build based around health degeneration and interrupts, and got the bonus rather easily. Thus I demonstrated the maddening depths of my considerable incompetence, and that once in a while, puny weapons are a match for superior intelligence.
Lesson the Third: Profanity improves morale. When two horny teenagers discuss sex in party chat, and a third party member uses the acronym STFU in an attempt to get them to keep their conversation relevant to the mission, the horny teenagers may take offense. The one that plays the monk may threaten to leave. To show your team spirit, respond with "I'm trembling in my boots." This will ensure the prompt disintegration of the party before you even reach the Ghostly Hero in Thirsty River. As a bonus, you may be rewarded by having the monk call you "lesbo" in private chat, and ignore you for the rest of your days. Consider that you accomplished all this by using a complete sentence, free of profanity and coarse language, which did not cast aspersions on his ancestry, question his competence, slander his reputation, or for that matter, directly mention him at all.