Edit, 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 on the Farmer Nishi page 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 slot 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 Sunlight casts from Siren while they're in the back row; more for Mechanicals and less for Critters.
Summary:
vs Siren:
1. Sandstorm
2. Crush
3. Deflection
4. Crush
5. Crush
6. Deflection
7. Crush
8. 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: this overall strategy versus Siren also works, and maybe saves 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.
(Mysterayne, I'm sorry you had a bad experience, but I've done this fight hundreds of times to level pets -- http://www.warcraftpets.com/account/profiles/Tale/ -- and her rotation is very consistent.)