If your substitute pet for the Ghost Pup has more HP than the Val'kyr (in my case, I used a Ghostly Skull), just start the fight with it first, get hit once to lower its HP, then proceed as described in the strategy.
Undead Team: Ghastly Kid (H/H) - 1, 2, 2
Infected Squirrel (H/P) - x, x, 2
Blighted Squirrel (H/P) - 2, x, 2. Any breed will work. Breeds with less health will work even better.
Ghastly Kid: 1 - Ethereal
2 - Haunt
Infected Squirrel: 1 - Corpse Explosion
Blighted Squirrel: 1 - Woodchipper
2 - Stampede
Ghastly Kid: 1+ - Hoof
It would be better if Blighted Squirrel dies for the first time before it starts Stampede because in this case Snozz will have damage increasing debuff (Shattered Defences) for 2 rounds when Ghastly Kid will come into battle. But don't worry too much if it doesn't happen - most of times Snozz will die after the first attack of Ghastly Kid. The second attack requires only if you are unlucky with RNG.
Alright, so after testing some undead pets I found a possible solution. Mind you rng will happen. I replaced Scraps with Spectral Spinner (1-2-2). Used the regular rotation. Once Spectral Spinner is brought in for the second time use Blinding Poison and then Spectral Strike. Hopefully you dont get hit through the blinding part.
It shouldn't really matter - Unborn Val'kyr is easy to get nowadays. There's one spawn up in every zone at all times, so just some flying around will get you one quick enough. Been leveling a char in Northrend and if 3 pets wasn't the limit I could have ten Unborn Val'kyrs by now.
I didn't have Scraps or Infected Squirrel leveled, so I swapped in Macabre Marrionette for the Squirrel and Bigglesworth for Scraps, to pretty much the same effect...
Start with Marionette
1: Pass, Val'kyr forced in
2: Curse of Doom
3: Unholy Ascension
-Bring in Bigglesworth-
4: Pass, Marionette forced in
5: Siphon Life
6: Dead Man's Party
-Marionette Dies, Bring in Bigglesworth-
It's a little RNG, but worked on my second try. It's essentially the same idea as the original strat, except using Siphon Life & Dead Man's Party as a substitute for Creeping Fungus & Stampede. And using Bigglesworth to close, as suggested in a post above.