Thundering Pandaren Spirit

2 pet strategy I found really useful on the Thundering Pandaren Spirit NPC page (by Posh)

The Thundering Pandaren Spirit. He was always one of the hardest battles I ever had, and I struggled all the time against him, until someone very kind shared with me their secret to the fight...

Now it is easy to 2 pet battle and win it every time with a leveling pet, even a level 1 pet can survive this battle

Team:

1st slot, leveling pet, (even level 1)
2nd slot Level 25 Snail/Whelk, (any kind), suggest anything but a B/B snail, Abilities: Absorb, Acidic Goo, and Dive,
3rd slot, Level 25 Mechanical Pandaren Dragonling (must have used a stone to convert him to rare), Abilities: Breath, Bombing Run, Decoy

Reasons to use Snail instead of ANY other critter such as a rabbit:
I have used many different critter pets (including S/S rabbit) till I discovered the snail. Dodging and swapping pets you loose so many unnecessary rounds.
I found the snail to be one of the fastest pet battles, because of the 25% damage debuff + dive (+damage against elemental pets.) After swapping in the snail, the spirit is typically dead in 3 to 5 rounds (Dive sometimes 1-shots him).
Then having the snail hit Sludgy before dieing with goo, lets the dragon hit 25% harder, so the slime typically only lasts 2 breaths. leaving the dragon to finish the mole very easily. It is a extremely fast battle, (except rarely when dive misses the elemental and forfeit and try again, you don't have too, the snail can still win, its just faster to forfeit)

Battle:
Round 1 vs Pandaren Earth Spirit, low level pet gets stunned, (if you use a critter, do not use an ability that has a chance to stun the Earth Spirit)
Round 2 switch to your Snail pet
Round 3 Goo him
Round 4 Dive
Round 5 If dive did not hit, forfeit, and try again, otherwise, Absorb till it is dead. (you can try to keep Goo up, but I did not find it worth maintaining)
Next round, vs Sludgy, Acidic goo him, and Absorb till dead, Use Dive if you can dive without dieing (you need over 400hp to Dive), because he will dot you before you go under, and you will die submerged, so it was better that you Absorb till dead (Try to keep goo up especially before you die)
When Sludgy kills your snail
Switch to the Mechanical Pandaren Dragonling, and just use breath on him, DO NOT WORRY ABOUT HP, because your dragon only needs to have 1hp to win against Darnak the Tunneler (3rd pet).
Round 1 with Darnak the Tunneler. Use bombing run,
Round 2, use Breath (Darnak will burrow after getting hit)
Round 3, use Decoy, because the Darnak the Tunneler will have used Burrow last round
Use breath till Darnak is dead, you will die, but revive, and finish off the Darnak.

I have only had the Mechanical dragon die once before... I always win with this strategy.
Another strategy involves using rabbits instead of snails, but I just use the snail strategy because I like the extra 25% dps, and the shorter rounds that the goo attack gives you.

-Posh

UPDATED for 5.3!
Here's a guide to beat this trainer with only 2 level 25 pets, allowing you to "carry" and power level a third!

Step 1: You will need a rabbit pet with >300 Speed at level 25. The best candidates are just over 300 speed, which is fast enough, while maximising the other stats. Certain breeds of rabbits will work best (Rare breeds 3, 11, 12). If hunting for the correct breed is not your thing, you can also use a [npc=32791], which will have enough speed even at uncommon rarity and is very easy to acquire. If, like me you are lucky enough to have [npc=59358], even better, as he is already rare!

Step 2: You need a mechanical with Beast attacks to finish off Sludgy. I have found the [npc=68838] works the best. I will write this guide using this pet, although it can be applied to other mechanicals - just be sure to select at least one strong Beast type ability. Easy to acquire alternatives include [npc=43916] and [npc=15699].

Step 3: Have your carried pet handy - place it in the first battle slot so it starts.

Here is the setup you want to run:
Slot 1 - [Carried Pet]
Slot 2 - [Rabbit of your choice] : [petability=360] | [petability=312] | [petability=159]
Slot 3 - [npc=68838] : [petability=429] | [petability=923] | [petability=536]

Enemy Team Summary
Pandaren Earth Spirit : [petability=569] | [petability=814] | [petability=801]
Sludgy : [petability=448] | [petability=450] | [petability=445]
Darnak the Tunneler : [petability=436] | [petability=159] | [petability=621]

Pandaren Earth Spirit

Below is the ability order that this pet uses against you.
1. [petability=569]
2. [petability=814]
3. [petability=801]
4. [petability=801]
5. [petability=801]
6. [petability=801]
RESTART AT STEP 1

As your rabbit is a critter, it takes reduced damage from Elemental attacks AND the Crystal Prison will really do nothing to the critter as CC is less effective against critters! You want to avoid [petability=814], since it hurts. The way to do that is as follows.

Use your carry pet to soak round 1 and 2*. Switch in your rabbit (round 3) and use [petability=159] for rounds 4-5 (You will avoid one [petability=801] and soak one the following round). Next use [petability=360] for two rounds (round 6 and then back to round 1), then use [petability=312] to avoid the [petability=814] in round 2 (and also the following [petability=801] on round 3!). Use [petability=360] for round 3, and then start again with [petability=159] on round 4-5 (as listed above), rinsing and repeating till the Earth Spirit is dead!

*NOTE: The [petability=814] in Round 2 as listed above will deal about 500-550 damage to your carried pet, or about 350 if they are a critter (takes less damage from Elemental attacks). So, unless they are Mechanical (will resurrect after taking a killing blow) or Magic (can only take a maximum of 40% of it's health in a single hit) you will want to make sure your carried pet has enough health to withstand a hit!

Sludgy

Mechanicals take less damage from 2 out of 3 of Sludgy's attacks, and when possessing a Beast type ability, can put out a lot of damage to get him down quick.

When Sludgy comes out, switch to your mechanical - in this case [npc=68838]. The strategy with the [npc=68838] is very straightforward - [petability=536], then [petability=429] till Sludgy is dead. It may well be that your mechanical dies or is very close to death by the time Sludgy goes down - that's ok.

Darnak the Tunneler

Below is the ability order that this pet uses against you.
1. [petability=436]
2. [petability=159] (Submerge)
3. [petability=159] (Emerges)
4. [petability=621]
5. No ability (all abilities on CD)
6. [petability=621]
RESTART AT STEP 1

Time to bring your rabbit back in to work his magic! The precise setup of abilities selected for your rabbit means that it will take ZERO damage from Darnak. Here's how.

Switch your rabbit back in as soon as Darnak comes out; he will do [petability=436] when you do so. Next use [petability=360] for one round, and when Darnak submerges, use [petability=312], which avoids not only the [petability=159], but also the following [petability=621] on round 4! Continue with [petability=360] for rounds 4 and 5 (round 5 is a free shot round, Darnak doesn't even do anything!) and finally [petability=159] for round 6, which avoids [petability=621]. Now rinse and repeat your way to a win!

As one of the hardest trainers out there, this method will be a sure fire way to farm experience and bags day after day.

Here's my strategy write-up for those who do these guys as dailies, and want an easier way of handling them, and this may be of use for those who are taking these guys on for the first time. I go for efficiency here - you'll see me recommending a lot of the same pet, because if you're doing these once a day, you don't want each one to leave you in a panic. The Pandaren Spirits are Legendary quality, so they have high survivability - but they're very easy to beat if you use a specific type of pet, so their attacks against you are weak and your attacks against them are strong. I advise 25 Rares, but if you have the right setup, you can live dangerously and go a few levels lower/use uncommon if you want.

The pets don't always appear in the order listed (trainers at time switch the order of their pets), but you can run from the fight and try again, or switch your own order of pets, if you'd like more choice in your set up. Eventually you'll get a line-up you're more comfortable with.

Pandaren Earth Spirit, Elemental - Crystal Prison, Rupture, Stone Shot
1769 HP, 300 Speed, 334 Attack
The Earth Spirit is predictable, because, although he starts out by CC'ing you, he uses standard Elemental moves after that, which are weak against Critters - and Critters also break out of that CC right away. So start with a fast Critter, preferably one with Aquatic moves (which Elementals are weak against) like a Rapana Whelk or any sort of Snail (WELL-worth the investment if you're doing the Spirit Tamers), and this dude will be down before you know it. If you do not yet have a Whelk, pick a fast Critter so you can get the first move!

Typical attack script pattern: Crystal Prison, Rupture, Stone Shot, Stone Shot, Stone Shot

Sludgy, Critter - Creeping Ooze, Expunge, Ooze Touch
1769 HP, 334 Speed, 300 Attack
When it comes to Sludgy, he's primed to beat the crap out Elementals and Flying-types, but his natural Beast predators have no immunity here, so you can feel free to whip out a strong Beast to tear him apart. But... if we want to be efficient, we should instead use a Mech with Beast abilities, since Mechs are resistant to 2 of his Critter abilities - an Anodized Robo Cub works great here. Just keep in mind that, when Sludgy dies and you're up against Darnak, the Robo Cub ain't gonna last long.

Typical attack script pattern: Expunge, Creeping Ooze, Ooze Touch, Ooze Touch

Darnak the Tunneler, Beast - Burrow, Stone Rush, Stoneskin
1975 HP, 300 Speed, 300 Attack
For Darnak, the toughest of the batch, you'll notice that he has two moves that makes him annoying as sin: Burrow (anti-critter, weak-flying) and Stone Rush (anti-mechanical, weak-critter). He typically starts with Stone Skin, Burrows, and then Stone Rushes you until you're on the floor crying/til Burrow's back up. Because he's a Beast type, you'd naturally be inclined to use a Mechanical against him - however, Stone Rush will hit around 1k damage, taking out your Mech quickly. And because Burrow is purely anti-Critter, that takes a Critter off the table. So a good pet here would be something that has Mechanical moves, but isn't itself a Mechanical. The most accessible pet I've found has been the Enchanted Broom, which has two solid mechs, as well as a anti-nuke racial. Alternatively, if you're one of the lucky few: Grunty would work great here. If you can't acquire either, another great option is a flying pet with high health and a strong nuke (such as Lift-Off, which will let you avoid his Burrow).

Typical attack script pattern: Stoneskin, Burrow (dig in), Burrow (burst out), Stone Rush, (Pass [no moves available]), Stone Rush

See my strats for:
Burning Pandaren Spirit
Flowing Pandaren Spirit
Whispering Pandaren Spirit