A few assumptions: First, I'm going to assume your Black Mage or Scholar is at or near level 80. This isn't particularly hard to do as I'd thought given Abyssean EXP groups. Spend an evening killing Mandies or whatever and get it done. Second, for the early portion of the trials you're going to need to Warp. Probably a lot. So it would be easier to do as Black Mage over Scholar if you've got both options. For the trials that require killing 200 of a mob and no BST pets are available, Black Mage is definitely preferable due to -ga3s. Really, aside from one-shotting BST pets I'd go with BLM over SCH for these. (Your mileage may vary, personal preference and all that.)
You can also Retrace yourself as Black Mage and that makes jumping between (S) zones a lot more convenient.
Second, I'm going to assume you aren't going to Level Sync. There's several trials that you could do via SMN burn or syncing below level ~50. However, not everyone has a mule they can dual-box or a friend with a job they can drag out to the middle of nowhere to do a weather trial. As for the SMN burn option on Cerberus at least K.tunnel is RMT monopolized and who knows if Square-Enix will suddenly decide to LM-17 people for SMN burning January 22, 2011 (or whenever).
Let's Get Started!
The Maps (click to enlarge):
Now you're the proud owner of two Teiwaz. Each with completely terrible stats and each flagged for your next two trials. Trial 791 which is killing 50 spiders under light-aligned weather and Trial 841 which is killing 50 slimes under dark-aligned weather. Unfortunately, both these purely weather-based trials suck. The second (usually) much more than the first. But this is why you're doing both at the same time...eventually. Toss your Dark Teiwaz into your Mog Satchel or Safe for the time being. We're going to kill some spiders.
Trial by Spider (791)
The gist of this trial is you need to kill 50 spiders under light-aligned (wind, fire, thunder, light) weather. Sounds easy enough. And if you happen to get three days straight worth of weather it's hilariously easy. Unfortunately, you most likely won't. God hates you and all that.
The practical zones are Kuftal Tunnel, The Boyahda Tree, and outside Whitegate. You can find out which would be your best bet by checking the weather in Cape Terrigan, The Sanctuary of Zi'Tah, and either zone outside Whitegate (they both seem to share weather in my experience). The NPC for each can be found in Selbina (on the roof of the building with the Sheep outside), at the exits to Port/Lower/Upper Jeuno, and in Whitegate in the same "cube" as the Sanction guy.
So to summarize your target zone planning phase:
Target Zone - Weather to check - NPC Location
Kuftal Tunnel - Cape Terrigan - Selbina
The Boyahda Tree - The Sanctuary of Zi'Tah - Port/Lower/Upper Jeuno
Outside Whitegate - Outside Whitegate - Whitegate (upstairs same "cube" as the Sanction dude)
So check and see which zone looks to be the best bet for Wind or Thunder weather. If possible, park a mule by the NPC. If you check and it's sunny for the next three game days I'd ignore that zone for now.
As far as getting there... Outpost Warp to Vollbow is your fastest way to Kuftal. If you have the Mini-Trial by Thunder Warp available it's by far the fastest way to The Boyahda Tree. I assume everyone can get to Whitegate in a timely manner.
More Weather Trials (816, 817)
So now you've gotten through by far the most aggravating part of your Indra's Staff trials. From here on out you can wait until Lightningsday, look for thunderstorms, or just need to have thunder damage killshots. Which is to say you can make definite, continuous progress, but welcome in Uncle Tedium. It's going to take a while.
For Trial 816 you need to kill 50 mobs during Thunder day/weather. If you want to know when the day will roll around, check out this Vanadiel Timer. (The thing you want is on the bottom right.) For thunderstorms the Boyahda Tree, Vunkerl Inlet, Caedarva Mire (think Chigoes by Azoup Isle staging point), Grauberg (chigoes!), and Sauromugue Champaigne (S) (beetle pets), and Xarcabard (S) (Gigas's Tigers) are all options.
Trial 817 is also rather easy. Kill 75 Beast family mobs with Thunder day/weather. The name of the game here is 's Tiger. Slaughter a bunch of Tiger BST pets and you'll be done in 1-2 game days solo.
Xarcabard (S) by the OP is great for this on Lightningsday (there's never thunderstorms there otherwise). Vunkerl Inlet has a decent spot with three Gigas's Tigers. You can only reach two of them from either of the two camps though unfortunately. The path there, assuming I'm remembering correct is this:
Multi-Tasking (Or: That Dark Teiwaz You Made)
Although I wouldn't recommend very highly it you can work on Trial 817 in Caedarva Mire off the Black Sheep by the Black Sheep NM near Azouph Isle Staging Point. This has the potential bonus that you can have both Water and Dark weather there and there's two Slimes (and a third if you fish it up).
Aside from that, put the Dark Teiwaz away again, you won't be needing it for a while.
Trial 822 (Or: Stop Being Sociable)
Until now you could team up to do these trials, but from here on out you need killshots. So unless you're teaming up with people who don't need killshots too, then you will be solo from here on out. Trial 822 is to kill 150 Vermin-family mobs.
You could go back to The Boyahda Tree and kill 150 spiders, but you'd probably rather kill yourself. The reasonable options here are -ga3ing Chigoes and killing BST pets.
Grauberg (S) has Chigoes where you did your first trial and you can -ga3 down 2-6 of them at a time. Just be aware that if you see a group of THREE Chigoes one is an NM that's going to go all WTFBBQKITTENRAEP on your face. I used red text, because it'll Firaga III your ass. So be on the look out for that. Chigoes have Additional Effect: Disease as well, so if you're /RDM for Phalanx you should try to keep Barvirus up to reduce its duration. Or splurge on Remedies, I guess. I'd just wait it out personally.
Alternative two, is to get a preview of your second to last trial for Indra's Staff, Trial 1682. Goblin's Beetles in Sauromugue Champaigne (S) are vermin and are very easy to kill. There's two at (J-7) and they die rather easily. (You could Aspir them too if you want, I guess.)
The bright side is that your kill shots now have some extra oomph and you're 200 BST pets and 450 "real" mobs from Indra's Staff. So let's move along!
Multi-Tasking (Part II) & Trial 823
Now you're going to need to bring your Dark Teiwaz along with you for the coming trial. You need to kill 200 Slimes with Thunder killshots. There's no BST pet Slimes (that I'm aware of) and you need killshots for your Thunder Teiwaz. So you can't really team up so well for this with other people doing the same trial. This is where the map above finally starts to come together again. Basically, camp where you can kill Slimes and hope for dark-aligned weather to pop.
This puts you in Garlaige Citadel (S) or Den of Rancor. If you're going to do the Shitadel (S), the camp is here:
Garlaige (S) only supports one person, so do a /sea before heading out.
Just remember at either spot if you see Earth/Water weather to switch over to the Dark Teiwaz and get kills on it.
That Dang Dark Teiwaz...Thing.
If you're like me, you'll still have plenty of kills left on your dark-aligned Teiwaz. You can take a break from your Thunder Teiwaz and try to grind it out or keep on going.
Trial 1682 (Or: I hope you liked Beetles the first time!)
This one's easy. Go back to Sauromugue Champaigne (S) and kill Goblin's Beetles. Check with /sea first to see if somebody's already there though. And put up a /seacom if you haven't been doing so by now.
The End is Nigh
Ah. The last trial until you've got an Indra's Staff. And it's going to take several more hours. Spirit crushing, right? Well, Trial 1683 isn't bad, provided all the camps aren't taken, it's just slow. You need to killshot 250 Funguars with Thunder.
Location:The Boyahda Tree; Death Cap; Near the entrance of the tree, there are 5-6 rooms full of funguars that can be grouped and killed via sleep-nuke with Thundaga III. It is recommended to use Phalanx and Wizard Cookies for their Plantoid Killer effect.The only other spots are the alleyways in Pashow (S) or Crawler's Nest (S) (on the second map in the round room in the southern central there's 7 pops). The Tree is my personal favorite of three, but it can be crowded. /sea is your friend.
Location: Grauberg (S); Brasscap; Same tactic as above. There is a group of 5-6 at (J-7), another group of 5-6 at western (K-7) and 3-4 more at eastern (K-7). Be careful of hitting campaign mobs and Pixies with AoE spells, and beware of low HP aggro from Undead at night.
Now go and finish up that dark-aligned weather Slimes trial! I'll try to get up Part II (and finish Varuna's myself) tomorrow or the day after.