Farmer Nishi

Edit, January 2023: While this is tried and true, it's slow compared to many others. As top comment and still getting the occasional upvote, it is my duty to inform you that there are better ways than this, my first ever extremely wordy pet battle write-up. Check out Xu-Fu's Pet Battle Strategies for not just this trainer but every other in the game. It's a great resource, with convenient import strings for use with the Rematch addon. This strat is listed there at a time of 4 minutes, for use with leveling pets 7+. I personally now use the [npc=85527] strat, originally by Thulsadoom below and reposted at Xu-Fu by Bösendorfer, at barely more than half the time of this one. With changes to how leveling pets get experience, that strat gives full credit to two pets as low as level 2, making it better in every single way.

Back to the original post ... (but personally, I think you should stop reading here, and click the link to go to Thulsadoom's comment and give an upvote).

[npc=68659] wrecks her team, leaving two slots for leveling. Set the Idol up 111, [petability=406], [petability=453] and [petability=490] and start the fight using it. I use only one leveling pet with a [npc=55356] in my third slot set up 112, [petability=777], [petability=646], and [petability=209], for a faster fight and maximum level gain of one pet. Low level pets should have at least 280 health to soak two possible Sunlight casts from Siren while they're in the back row; Mechanicals will need even more health and Critters can get away with a little less (I'm not sure of exact numbers).

Summary:
vs Siren:
1. Sandstorm
2. Crush
3. Deflection
4. Crush
5. Crush
6. Crush
7. Deflection
8. Crush
9. Crush
... and restart from step 1 if Siren isn't dead yet.

vs Toothbreaker:
1. If Sandstorm isn't up, put it up. Otherwise skip to step 2.
2. Swap to low level pet
3. Swap back to Idol
4. On the turn where Sons emerges, use Deflection
5. Crush until Submerge
6. Sandstorm
7. Go back to step 4, passing until emerge.

vs Mothallus:
1. Crush
2. Pass
3. Deflection
4. Swap to Tonk
5. Shock and Awe
6. Ion Cannon if Mothallus is < 1000 hp (usual case), otherwise:
6a: Missile
6b: Ion Cannon

Long form:

[npc=66718] always opens with [petability=404], so your immediate first round counter is Sandstorm. Her second cast is always to get the heal going, [petability=268], so Crush on the second round. Then [petability=753] will always come third, so Deflection it away. Siren then has to recharge for two rounds, so Crush until Deflection is off cooldown and use Deflection again, which will catch the next Solar Beam. She'll have two rounds of recharging, so Crush for them both then on the third round, if Siren is still alive, you'll see 1 cooldown round left on your Deflection. This round she restarts her opening order, and you should too: Sandstorm to counter her Sunlight, then Crush as she renews Photosynthesis, and Deflection to catch Solar Beam. Usually she'll be dead before that third Solar Beam, though, unless you were really unlucky with Crush misses.

(Note: you can maybe save a turn or two if you don't bother deflecting the second Solar Beam and just take the hit. It just takes a little more attention to be sure of where you are vs. Siren's next Sunlight cast, so you can counter it that round with Sandstorm.)

Next up is [npc=66716], who immediately submerges with [petability=828]. Swap in your low level pet who should take no damage at all from the Sons thanks to Sandstorm, unless the pet is an elemental which ignores the weather. You can either swap right back out, or have the low level pet do one action -- which will guaranteed miss Toothbreaker, so it's a nice bonus if it is something that buffs your team without having to hit, or it lays a trap. Then bring back the Idol, who will get hit with the [petability=745], but it really doesn't matter so don't waste a turn deflecting it. I recommend delaying a refresh of Sandstorm until the next submerge, because with clear weather your Crush has improved hit chance and damage, and Toothbreaker isn't doing meaningful damage to you.

The path from here depends a little on whether your second leveling slot is also a low level or is a 20+ pet who can finish off the fight. I personally prefer the latter, using my Tonk, but mathematically you get the same XP (split across two pets) if you use two low levels. You get the least combined XP for a low level plus a 20-24 pet. If you do use two low levels, then time the swap with another submerge. Finish Toothbreaker with Crushes.

When [npc=66715] comes out, Crush the first round and then he [petability=159]s. Nothing useful you can do in that second round, so just pass. In the third round, counter the Burrow hit with Deflection. If you used your two leveling slots on low levels against Toothbreaker, then just slowly finish out the fight with Anubisath. Otherwise, after beating Brood down as far as you can without killing it, let Brood kill the Idol and bring in your 20+ pet to finish off the fight. The Brood's attacks are strong against Undead, Critter and Humanoid, weak against Flying and Aquatic. As a Beast it is vulnerable to Mechanical attacks and resistant to Humanoid attacks.

This is an updated post for 5.2. While this trainer is easy to beat with 2 pets (or even one for that matter), the constant healing and such can lead to a somewhat longer-than-necessary battle. I've found what I think is probably the absolute fastest you can hope to beat this trainer.

The strategy revolves around good timing with [npc=61088] or any equivalent pet with Water Jet, Cleaning Rain, and Pump (there's a lot of such pets out there). Note that there are 2 breeds of Eternal Strider, and unlike most strategies, the stats on this pet may actually matter as the numbers are repeatable but close in terms of killing the enemy pets. The breed required is the 273 attack 305 speed breed; if your attacks are not killing the enemies at the times this strategy says they are, then you need a higher attack pet. Also, you must be faster than the 2 enemy plants for this strategy to work with this level of speed.

Enemy pet 1) Enemy starts with his Sunflower. Start with your low level pet. Attack once; you'll get hit by Sunlight. This is the only time your low level pet should get hit. Then, swap to your Strider.

A) Use Cleansing Rain; this strengthens your attacks and mitigates the enemy's healing ability.
B) Charge Pump
C) Cast Pump, does damage
D) Charge Pump again
E) Attack with Water Jet, finishes the enemy off

Enemy pet 2) Enemy swaps in the Turnip.

A) Cast Pump, which was charged from the previous pet. It hits him before he burrows, which he does on this turn.
B) Charge Pump (enemy is burrowed)
C) Cast Cleaning Rain (enemy is burrowed)
D) Cast Water Jet; combined with the damage they took from the first pump, this should kill them.

Enemy pet 3) Enemy brings in the worm

A) Cast Pump, which is charged from the previous pet. Enemy hits you with Goo.
B) Charge Pump, enemy Burrows
C) Cast Pump. This is where it gets a bit random. Normally the enemy's burrow will hit you and bring you down under 100 health. While it is very rare to die from it, a crit will push you over. If you don't die, the Pump will kill the enemy, game over. If you do die, just bring out any mechanical pet to finish off the last 800 damage.

Two Pet Carry

25 Rare Syd (Water Jet, Bubble, Cleansing Rain)
Two Carry pets (250 health min)

Open with Syd and Cleansing Rain, Bubble. Spam Water Jet till Siren dies.
Enter Toothbreaker. Swap to Carry pet #1, Swap to Carry pet #2. Swap back to Syd, Water Jet (x3), Cleansing Rain, spam Water Jet till Tooth breaker dies.
Enter Brood. Water Jet, Bubble, Spam Water Jet till Brood dies.

21 rounds