Found in Crystalsong
Info taken from comments on WoWhead.com
Your best bet against this battle is a speedy rabbit and a crab. Set your rabbit to have Scratch, Dodge and Burrow, and set your crab to have Snap, Renewing Mists, and Shell Shield. Cancel the fights until you get Spooky Strangler first.
Open with Dodge to avoid taking Curse of Agony, followed by Burrow to avoid Curse of Doom. After your rabbit attacks he will swap your pets around. Hopefully you’ll get landed with the crab (I was always fortunate to have the rabbit replaced by the crab.) Use Renewing Mists, then switch back in your rabbit. Attack with Scratch, he will land Curse of Agony on you, but Renewing Mists should hopefully help you recover some of the damage taken as result. Use Dodge now to avoid Curse of Doom, then attack with Scratch. He will swap your pets around again. Hopefully he is near death (unless you had a bunch of unlucky misses, in which you will then want to switch the rabbit back right away and use Burrow). Use Snap 1-2 times to knock him out once. When he resurrects apply Renewing Mists and he’ll just fall dead.
Enter Stitch, luckily his attacks are weak to your crab. Apply Shell Shield and continue a rotation of Snaps, Shell Shields and Renewing Mists whenever they wear off until he is down.
Enter Mort. Try to do whatever damage you can to him before he kills you. He will usually apply Siphon Life, followed by Spectral Strike and then a powerful Ghostly Bite. Re-apply Renewing Mists before he does his Ghostly Bite. Your crab will get owned, but those Renewing Mists will help keep your rabbit alive for a few rounds. Switch in your rabbit and use Scratch, followed by Dodge and then Burrow. Hopefully this will knock him out once. Use this opportunity to switch in your leveling pet. Mort will die before you can attack, but your leveling pet will then gain a nice bit of xp.