http://guildwars.wikia.com/wiki/The_...feating_Mallyx
Team 1: W/Rt, E/Me, N/Rt, Mo/E
Team 2: W/Rt, E/Me, Rt/Me, Mo/E
Builds:
[build prof=W/Rt name="Team 1 Ursan Warrior" box str=12+1+1 axe=10 chan=8 rest=2 desc="Weapon #1: Zealous Axe +5 Energy +30 Health{br}Weapon #2: Staff +20 Energy + 30 Health"][Agony][Destruction][Bloodsong][Light of Deldrimor][Lightbringer's Gaze][Gaze of Fury][Flesh of My Flesh][Ursan Blessing][/build]
[build prof=W/Rt name="Team 2 Ursan Warrior" box str=12+1+1 axe=10 chan=8 com=2 desc="Weapon #1: Zealous Axe +5 Energy +30 Health{br}Weapon #2: Staff +20 Energy + 30 Health"][Pain][Bloodsong][Agony]["By Ural's Hammer"][Lightbringer's Gaze][Gaze of Fury][Flesh of My Flesh][Ursan Blessing][/build]
[build prof=E/Me name="Team 1 Air Elementalist" box air=12+1+2 energ=12+1 insp=3 desc="Weapon #1: Staff +15 Energy +60 Health"][Glyph of Lesser Energy][Mind Shock][Lightning Javelin][Shock Arrow][Lightning Bolt][Lightning Strike][Ether Signet][Leech Signet][/build]
[build prof=E/Me name="Team 2 Air Elementalist" box air=12+1+2 energ=12+1 insp=3 desc="Weapon #1: Staff +15 Energy +60 Health"][Glyph of Lesser Energy][Invoke Lightning][Lightning Javelin][Shock Arrow][Lightning Bolt][Lightning Strike][Ether Signet][Leech Signet][/build]
[build prof=N/Rt name="Team 1 Necro" box soul=12+1+2 rest=12 desc="Weapon #1: Staff +15 Energy +60 Health"][Spirit Light Weapon][Vengeful Weapon][Wielder's Boon][Ghostmirror Light][Protective Was Kaolai][Gaze of Fury][Mend Body and Soul][Flesh of My Flesh][/build]
[build prof=Rt/Me name="Team 2 Ritualist" box rest=12+1+2 spawningpower=12+1 insp=3 desc="Weapon #1: Staff +15 Energy +60 Health"][Consume Soul][Empowerment][Wielder's Boon][Ghostmirror Light][Protective Was Kaolai][Gaze of Fury][Mend Body and Soul][Ether Signet][/build]
[build prof=Mo/E name="Team 1 Monk" box heal=12+1+2 div=12+1 desc="Weapon #1: Staff +15 Energy +60 Health"][Glyph of Lesser Energy][Healing Ribbon][Heal Other][Word of Healing][Cure Hex][Dwayna's Kiss][Signet of Rejuvenation][Resurrection Chant][/build]
[build prof=Mo/E name="Team 2 Monk" box heal=12+1+2 div=12+1 desc="Weapon #1: Staff +15 Energy +60 Health"][Glyph of Lesser Energy][Healing Ribbon][Heal Other][Word of Healing][Purge Conditions][Words of Comfort][Signet of Rejuvenation][Resurrection Chant][/build]
What we did was just stay up top and kill the first wave (Margonites), and then the second wave (all but one Fury). Set all the Heroes to 'Avoid Combat' when only the two Furies are left so you don't accidentally kill them both. With one Fury alive, park Team 1 up the hill to the west along with the E/Me from Team 2, and then the W/Rt from Team 2 can flag his Rt/Me and Mo/E around the Priest Zhellix and make him attack the Fury while you drag Zhellix down the hill toward the gate. Relocate the rest of the party near Zhellix before the third group spawns, since they come from the West.

Then just park your whole party slightly up the hill and lead with the Ursan Warriors, carefully aggroing one group at a time and blocking the groups with the arena columns as much as possible. Slog through the 18 groups, taking care to only aggro one group at a time. Again, it's critical to know which kill will spawn the next group, and leave that foe alive until you're ready for the next group. You'll get two spawns on top of you on the hill, Group 9 and Group 17. Group 9 has two TortureWebs in it and can be quite damaging so dispense with them first, and quickly. If they kill a few heroes, you can always run further west to drop aggro. Group 17 is just Torments, and is not much of a problem. After the gate opens, head in to fight Mallyx.
In a dozen or so tries while we tweaked the build, our best time to gate open was 36 minutes. We didn't use any consumables before the cut scene. Once inside, we buffed up and each flagged our teams between two spirits. Using all four Gazes of Fury, take all the spirits down immediately. They come back up fast but GoF recharges in 20 secs. Keep casting GoF and Consume Soul with the Heroes while the Ursans fight Mallyx. When the Margonites spawn at 80% Mallyx health, leave Mallyx alone and kill the Margo's in the right order (Ki first, etc.). Keep the spirits down as required. Same process when the Torments spawn at 50% Mallyx health. Once Torments are down, kill Mallyx.

Notes about build:
-There is not a single enchant, hex, or condition causing skill on any bar (Except Ursan Roar - don't use on Mallyx).
-Warriors both have Sentinel insignias, with 14 Str and 10 Axe so mimimum requirements are still met while inflicted with weakness.
-Ele's do fine with regard to energy and exhaustion despite not having an Attunement (make sure they don't have Blessed insignias).
-Necro is used because of constant energy supply, and another rez.
-Consume Soul on the Rit is an awesome heal skill for the casters (212 Heal for all within the area). You can use Consume Soul on the Rit's Empowerment Spirit for healing the backfield.
-Empowerment and the Weapon Spells have great synergy with the Rit heals (Spirit Light Weapon, Wielder's Boon, Ghostmirror Light, Mend Body and Soul, etc.).
-Disable Gaze of Fury before the gate opens to keep the Ursan Warrior spirits up when they are out of Ursan and luring.
-We tweaked the build a little since the screenshot, if you're wondering about any differences (Signet Of Creation proved not necessary).
Mallyx WHO??!?
Have fun!
JD