Cassandra Kaboom

Cassandra Kaboom is the last Pet tamer you need to defeat before returning to Varzok in Orgrimmar. It's advised to have your three pets to level 12-14 before you fight her as all her pets are level 13.

Cassandra uses an all Mechanical Pet setup including pets Gizmo, Cluckatron, and Whirl. For your setup you do not want to use Beast pets as all of these Mechanical foes deal high damage against them. The best pets to use against her setup are all Elementals as her pets deal the least amount of damage to them and Elementals pack a punch against Mechanicals.

For the fight itself; Cassandra will usually start with Gizmo or Cluckatron, saving Whirl as a last resort for a good reason.
As with all Mechanicals; once they are defeated, they are then revived at low health, making you work a bit extra to take them out. Gizmo will often use Demolish which has a very high chance to miss it's target while Cluckatron will often use Toxic Smoke to put a DoT on your pet that can deal some mean damage with the wrong set up. Once you've defeated these two Cassandra is done messing around and she'll pull out her final ultimate weapon; Whirl. Whirl is a mechanical squirrel type and is not a pet to be disregarded.
A core reason Cassandra saves Whirl for last is due to the fact of his first ability; Batter. Since you've defeated her first two pets, you gain the right to attack first most of the time. But Batter allows Whirl to attack 1-2 times with an additional hit if your pet is the first to attack in this round. Avoid using weather abilities that increase healing done as well as increase the amount of HP all pets have on the battlefield while the effect is active, because Whirl will go nuts over it's HP gain ability; Repair. Which if Sunshine weather effect is present; will heal his HP full up over 1k HP. Even without a heal increase effect; Repair is still a crippling move that could devastate your battle-worn pets over and over if used repeatedly.
Whirls third ability is equally traumatizing to your pets as it increases the damage of Whirl's next attack by 150%. So be wary when you see his buff pop up, and if your pet has a shield ability; it'd be wise to use it for a devastating Batter.

Ravij's Ideal Setup for this Fight: Frigid Frostling, Pheonix Hatchling, Dark Pheonix Hatchling.

Here is my Quicklist Guide to the Horde Side of the Pet Trainer Intro Quests. This is a quick reference to see what type of pets you will need to defeat the trainers easily. On Trainers who have the same type of pet for all 3 pets in their party, I would recommend you bring at least two pets that are the opposite type (i.e. bring three appropriate leveled Mechanical types to fight three trainer Beasts). Don't rely on fighting all three of their pets with just one of yours, despite the fact that they will have stronger attacks and defeat them quicker.

1st Tier Horde Trainer Questline (Starts in Kalimdor)

To defeat these trainers with maximum efficiency, you will need pets of the following type:
Mechanical, Beast, Flying, Magic, Aquatic, Elemental

Zunta – Durotar
Quickteam List – Minimum Level 2 Mechanical, Beast
Pet 1: Spike – Level 2 Beast – Use Mechanical
Pet 2: Mumtar – Level 2 Critter – Use Beast

Dagra the Fierce – Northern Barrens
Quickteam List – Minimum Level 3 Mechanical, Beast
Pet 1: Springtail - Level 3 Critter – Use Beast
Pet 2: Ripper – Level 3 Beast – Use Mechanical
Pet 3: Longneck – Level 3 Beast – Use Mechanical

Analynn – Ashenvale
Quickteam List – Minimum Level 5 Flying, Beast, Magic
Pet 1: Mister Pinch – Level 5 Aquatic – Use Flying
Pet 2: Oozer – Level 5 Critter – Use Beast
Pet 3: Flutterby – Level 5 Flying – Use Magic

Zonya the Sadist - Stonetalon Mountains
Quickteam List – Minimum Level 7 Mechanical, Beast
Pet 1: Acidous – Level 7 Beast – Use Mechanical
Pet 2: Constrictor - Level 7 Beast – Use Mechanical
Pet 3: Odoron - Level 7 Critter – Use Beast

Merda Stronghoof – Desolace
Quickteam List – Minimum Level 9 Aquatic, Beast, Flying
Pet 1: Ambershell – Level 9 Elemental – Use Aquatic
Pet 2: Rockhide – Level 9 Critter – Use Beast
Pet 3: Bounder - Level 9 Aquatic – Use Flying

Cassandra Kaboom - Southern Barrens
Quickteam List – Minimum Level 11 Elemental
Pet 1: Whirls – Level 11 Mechanical – Use Elemental
Pet 2: Cluckatron – Level 11 Mechanical – Use Elemental
Pet 3: Gizmo – Level 11 Mechanical – Use Elemental

*Author's Note: In writing this guide, I used various references from wowhead and warcraftpets.com. I did not write this guide as I was doing each trainer. Some information might be wrong. Please feel free to comment and I will change it as needed.

Ok I too was unable to fight her after abandoning the account wide quest. What fixed it for me was to go to talk to [npc=66372] and she gave me the quest to fight again. Hope this helps someone!