Good luck on fighting Fenrir at 56, you'll either need some really good friends or a really big bribe. (Not exactly a safe and easy fight, even if everyone's 75 and you bring an "ideal party setup", someone lower level makes it that much riskier. It's not just a case of high level snobbishness.)
As for the details, however, it goes something like this:
1. Ya start the quest with the annoying taru who has to emote after every last line of dialogue, and thus makes it take two or three minutes just to get through the dialogue to start the quest. You get a key item, "Tuning Fork of X", where X is the name of an element (Earth Lightning Water Fire Ice Wind).
2. Ya make your way to the protocrystal in the cloister that matches your tuning fork. Your party makes its way to the protocrystal. Chances are, you're making your way to the protocrystal *with* the party. But never mind that. However you do it, you're all there, and you all have the matching tuning fork.
2.5. You punt the person who forgot their tuning fork, and send them off to go get it while the other five of you sit around chatting and occasionally seething.
3. One or another of you clicks on the protocrystal to initiate the BC, and then the rest do, and you go in, and you fight the BC. It's much like the solo battle, except there are bigger numbers involved (higher levels, more damage dealt and received, full party rather than one person).
4. With any luck, you win. Following the cutscene after your victory, you lose your tuning fork, and get a key item "Whisper of Y", where Y is an element name, but this one matches the cloister rather than the name of the element itself (Cloister of Tremors -> Earth Protocrystal -> Tuning Fork of Earth -> Titan Prime -> Whisper of Tremors).
5. Chances are, you're gonna be fighting more than one avatar, so repeat the process for all of them that you're fighting. It's generally best to get your tuning forks in advance, but there's Always Someone who didn't bother doing that. Just don't let it be you. XD
6. If you're not saving your whispers for Fenrir, then to get your actual *reward* (and the title, and the quest marked as completed), you go back to the original emote-happy taru NPC and talk to them with the whisper in your inventory. Note that if you *are* saving your whispers for Fenrir, you do not want to do this, because you'll lose your whisper. (If you do it accidentally, d/c yourself mid-cutscene.) You will be given the option of:
- A pact with the avatar, *IF* a. you do not already have a pact with the avatar and b. you have completed the quest to unlock the SMN job. You do not have to talk to the NPC as a SMN, you do not have to fight the avatar as a SMN, but you do need to have unlocked the job. As for the other requirement, it's obvious, no point in making a pact you already have.
- A miscellaneous rare-ex "weird item", which is for the Norg quest "Mama Mia" (from the uber-lispy Mamaulabion), which has as a reward, the Evoker's Ring (rare-ex, level 71 SMN, stats with -perpetuation and +skill if I recall). You need the "weird item" from all seven avatars - including Fenrir - to complete the quest. These are, incidentally, cameo items, each from a different FF game from FF1 through FF7.
- A rare-ex weapon, that may or may not be useful.
- A rare-ex ring, that's only useful one day of the game week.
- A rare-ex belt, that's only useful one day of the game week.
- 10,000 gil.
You do *not* get the option for an Earth Spirit, or Lightning Spirit, or any of the other spirit pacts. If you want those, you have to get it from its usual source (normal enemy, NM, BCNM, purchased, it varies), or buy it off the AH from someone else who got it from its usual source.
Anyway, after you get your reward, wait until 0:00 JST passes (that's 11:00 EST, 08:00 PST), and you can talk to the NPC for yet another emote-filled speech and a new tuning fork so you can do it again. You can repeat the process as much as you like, though you'll eventually run out of rewards to get (other than gil). Note that most of the rare-ex equipment is pretty unimpressive, though the accuracy-on-lightning-day ones from Ramuh are decent.
7. If you *are* saving your whispers for Fenrir, ignore step 6 entirely, except for that bit about d/c'ing yourself if you accidentally talk to the quest-giving taru. Instead, collect *and do not turn in* all six whispers from all six avatars. Once you have all six, go talk to Leepe-Hoppe on the roof of the Rhinostery in south Windurst Waters, to trade them in for a Moon Bauble key item. (You need to start the Fenrir quest first, which will require some pretty hefty Windurst fame. You can do this before or after collecting the whispers.)
Once you get your Moon Bauble, you're no longer carrying the six whispers around. If you want more tuning forks so you can go do the original avatars in future, just go back to the original tarus, and in a blessedly short speech which *isn't* filled with emotes, they chastise you for "losing" it and give you another one. (Any time after that, of course, you're turning in the whisper as for step 6 above, and you have to sit through their extended speech the next day.)
8. For Fenrir, the process is pretty much the same. Get your party together of people with Moon Baubles, go to Full Moon Fountain inside Toraimarai Canal, and smite. The "and smite" bit is, of course, the hard part, but I'll just gloss over that detail for now. After the fight you receive a key item "Whisper of the Moon".
And anyone who's gotten Windurst rank 10 (whether they've since changed nations or not) will also get a key item "Dark Mana Orb". It's useful for one thing and one thing only, and that's a final cutscene with the Star Onion Brigade after you finish their seventh quest. Star Onion Brigade quests and Windurst missions tie in with each other and with Fenrir, if you were wondering why this was Windurst-rank-10-only.
9. Go back to Leepe-Hoppe with your Whisper of the Moon and you'll similarly get a list of possible rewards:
- Pact with Fenrir (same restrictions as with any other avatar pact)
- "Weird item" for Fenrir, in this case it's the Ancients' Key (from FF7)
- Fenrir's Torque
- Fenrir's Earring
- Fenrir's Stone
- Fenrir's Cape
- 15,000 gil
(Fenrir's rare-ex items are, unlike the other avatars' rare-ex items, largely pretty decent. Earring for DD, Stone & Cape for tank, Torque for mage. They all do one thing during the day and another during the night. As opposed to one thing on one day of the week and nothing of note the other seven days.)
10. You do *not* have to go through that whole rigmarole again to get another Moon Bauble after turning in your Whisper of the Moon. Just like getting a new tuning fork after turning in a normal whisper, you just have to wait until 0:00 JST passes, and talk to Leepe-Hoppe for a new Moon Bauble.
