2. Goals and basics
Why go there? For EXP of course! For the fastest EXP for levels 71-85! Reward for completing Gods Rebirth is almost 2 mil EXP (although i'm pretty sure it's level based, anyways it's really a lot). It takes about three hours to do a full run.
How does it work? Get a full squad (6 people) of 71-85 level and gather at the Spiritual Disciple in one of the major cities (Archosaur 571, 646; Dreamweaver Port 661, 376; Thousands Streams 131, 857). Squad leader talks to the Spiritual Disciple then the squad is teleported to the special instance where the leader should activate another quest. The leader will need six Rebirth Orders Gamma (one for each squad member). Right after the activation of the main quest, show begins. The goal is to defend a NPC - Harpy Wraith that's located behind you from the monsters. First wave comes in 30 seconds and you meet them at the entrance to the valley. Monsters come in waves. Each wave brings mobs of one element in this order: Metal, Wood, Earth, Water, Fire. After the fifth wave of mobs a boss will come. To make things easier lets call these five waves + boss - a Stage. We have nine Stages. You will get EXP after each stage(after the boss is killed). Ordinary mobs there give 1 EXP 1 SP. To help the squad in the fight the leader can set bonus Aura's. Next to the quest NPC there are five Revelation NPC's, each of them can set an aura (around Holy Eye at the canyon's entrance) according to it's name: increased HP regen, MP regen, Chi regen, Defense, or Attack. The aura's also effect Venomancer pets. Setting aura's costs some beans (sort of local currency ) which is acquired by doing quests, from chests in non-battle area, for killing mobs in non-battle area, and from killing bosses. Each level (there are 6 of them) of an aura increases it's efficiency and costs more beans. A bit more about aura's and the order to set them in section 8. The mission is accomplished when the 9th (last) boss dies. EXP will be rewarded after each boss, each giving more EXP than the last, so even if you can't do a full run you will be rewarded.
Nice bonuses:
-Mirage celestones, Arcane Book fragments, level eight soulgems, and of course some pretty shiny coins.
-No EXP loss if you die
-Venomancer pets can level there
-PK'ers can get rid of their red names faster

3. Getting ready
You have to prepare yourself before you go to war, even before you gather people for your squad. So lets see what you'll need there.
Each squad member must have these items:
a) HP potions and Elixirs of your level( red and purple pots level 70 or 80)
b) Charms. HP charms on all squad members. MP charms for cleric, wizard, and archers
c) It is strongly recommended for archers to bring some MP pots. There are some mana draining mobs that can cancel archers AoE.
d) Venos and Barbs have to get some Teleport Incenses (boutique item, 10 silver) to return to the respawn point after quests (7-8 items) and opening chests (1-2 items)
e) Wizards and clerics are advised to have magic attack charms. They're really helpful on the double wave
f) For archers - Ammo!!!
Now about Remedies:
a) Archer, wizard, and cleric must have Dew of God Protection (absorbs 3000 damage for 30 seconds) to be used before the double wave
b) Barb and BM must have some defensive items. The main one is Tranquilizing Orb (8 seconds of invulnerability). For less dangerous situations Dew of Star Protection (greatly reduces all damage for 20 seconds).
c) Chi: There are two options here. Golden Jade (100 chi = 1 spark) or God's Tea (150 chi = 1.5 sparks)
d) Not that important but it's recommended for Barbs and BM to get some Nature Powder (immune to stun for 12 seconds) or Vacuity Powder (immune to stun for 20 seconds) for the last boss.
Oh, and don't forget about Rebirth Orders Gamma . You can get them at the Spiritual Disciple by trading your Duty Badges.
A few more notes before you start.
1. Make sure you have enough time. A full run takes about three hours of non-stop action so..
2. Eat, drink, smoke, turn off your phone, take your dog out, and visit the toilet yourself because you can not pause Rebirth. Later when you're more familiar with Gods Rebirth you will know when you can take a little break and when you can't.
3. If you make the bonus squad (barb, bm, archer, cleric, wiz, veno) make the BM the squad leader. This may help to avoid the glitch when you can't get your bonus beans.
4. Repair your equipment. You can do it inside but you might not live long enough to do it if your armor breaks in the middle of a fight

4. Squads, classes, and builds
Cleric: Either pure mag or mag/vit hybrid. Light armor is better at the beginning (especially if you have inexperienced barb and BM) because there are plenty of physical mobs in the first stages. During later stages light armor can be at least as good as other builds if you have really good equipment refined and sharded. For pure mag builds things are hard all the way (manly because of lack of HP). Hp shards in equipment is strongly recommended for pure mag builds.
Skills: Regeneration Aura - max, Purify 5+ if you have one DD and 7+ if you have two DDs to take care of at the end of the second Stage, Squad buffs - max, and max heals of course.
Venomancer: Any build works. Herc is recommended. Build doesn't matter as veno's are not required to do damage (would be nice to do some though) and Amplify Damage or Befudding Mist can be used by any build. Veno's also don't take much damage though sometimes die from AoE at the last boss, trying to cast Amplify Damage.
Skills: Parasitic Nova - the higher the better as paralysis time increases, Befudding Mist -lvl 10, Amplify Damage - lvl 10, Bramble Guard, Summer Sprint, and Soul Transfusion.
Wizards: Any build. Almost the same as cleric but since wizards get hit more it's hard for an arcane build, high level hp shards may help, but that's why light armor builds are preferred. Of course quality of equipment and refining is crucial. It's good for light armor builds to have their weapon refined +4 at least.
Skills: All basic single target spells for bosses, Dragons Breath - max, Fire Mastery - max, Stone Barrier - max, Black Ice Dragon Strike - might be useful against fire mobs.
Barbarian: Heavy pure tanks only. Classic tank with max hp and max defense. Need at least 10k hp.
Skills: Flesh Ream - max, Alacrity of the beast - to cancel bosses AoE, Talk is Cheap, HP and Attack buff - max, Slam - to push mobs into mages AoE, Sunder - max recommended.
Blademaster: Heavy axe/polearm build. Axe BM is required because they have higher DPS and stun skills(which are agro too). Amount of HP is vital. You don't want to die after using mass agro skill, right? Polearm BMs are good at moving mobs inside mages AoE. Unfortunately Polearm BM can't replace Axe so only if they're the second BM in the squad. Fists/claws BMs are useless in Gods Rebirth. Swords can also be the second BM in the squad. Myriad Sword Stance may help sometimes.
Skills: Heaven's Flame - BMs main skill here so - max, all other Axe AoE skills, Atmos Strike (sword skill) and Meteor Rush (polearm skill) needed to push mobs into AoE - 1 lvl at least, Axe Mastery - max, Golden Bell - max, Will of Bodhisatva would be nice for the last boss. Put sword, polearm, and Axe in skill bar to switch quickly between weapons.
Archer: Light Armor. Pure dex is welcomed but vit/dex hybrid works as well. Equipment should have hp gems and the weapon should be refined. Remember that Archer is the main DD there so you can draw your own conclusions...
Skills: Bow Mastery - max, Sharpened Tooth Arrow for the last boss - the higher the better, Barrage of Arrows - at least level 6, better to make it higher but keep in mind that higher levels will drain more MP, and at level 7+ two MP regen Auras might not be enough.
Common squads:
Cleric, BM, Archer, Barb, Veno, Wizard: Classical bonus party. Besides almost perfect balance this squad gets a bonus of 600 beans at the start which allows you to get one Aura. Usually MP regen to save charms but it depends on the squad. This squad has a chance to complete Gods Rebirth at 75+ and at 80+ it shouldn't have any problems. You will get all beans and finish all quests before you kill the Stage 2 boss, thanks to the Veno.
Cleric, BM, Archer, Barb, BM/Barb, Archer: One of the strongest squads. You won't get any bonus' at the beginning but even at level 71 this squad has a chance to complete the mission. Squad does higher DPS than a squad with wizards and the backup of two Barbs or BMs helps to avoid many deaths. Beans and quests usually finished by stage 3 or 4.
Cleric, BM, Archer, Barb, Wiz/Archer, Archer: Another strong squad build. High DPS and survivability at lower levels.
Cleric, BM, Archer, Barb, Cleric, Wiz/Archer: Two clerics party is exotic but can complete it as well.
Cleric, Archer, Barb, Wiz/Archer, Veno, Veno: Another exotic build.
To simplify squad builds and avoid writing every single squad build here. Every squad should have at least two DDs(Wiz, Arc; or Arc, Arc). Never take two Wiz as the only DDs in the squad. Chances to get past Stage 1 is almost zero (depends on level and equips tho). Also don't try to make a squad without a cleric, Barb, BM, or Archer.