Critical Critters

Against Gnasher, I used:

• 
  
• Cockroach (1,1,1)
• 
  
• Egbert (1,1,1) - any pet with bite should work. Mine had 276 speed, so the boost from hiss made him faster than Gnasher.
• 
  
• Borean Marmot (1,1,2) - any pet with burrow and an ability comparable to chomp (213-396 damage) may work
• 
  

Strategy by Round

1. 
   
2. Start with Cockroach. Hiss
3. 
   
4. Swarm
5. 
   
6. Swarm continues
7. 
   
8. Swarm continues.* Cockroach dies, bring in Egbert (or another pet with Bite)
9. 
   
10. Bite
11. 
    
12. Bite. Egbert dies, bring in Borean Marmot
13. 
    
14. Burrow
15. 
    
16. Burrow hits
17. 
    
18. Chomp
19. 
    

*if your cockroach dies in round 3, skip to step 5

I tested this strategy twice - the first time, swarm got through three rounds, the second time it only got through two. Both times, the fight ended with burrow and then one chomp from Borean Marmot after Egbert died.

Against [npc=128019], I used

[npc=98506] 1,1,2
[npc=107206] 1,1,2
[npc=87257] 2,2,2

1. Smoke Bomb
2-4. Stampede
5. Scratch
6. Pass. Dust Bunny dies. Switch in Zoom
7-8. Slow and Steady
9. Charge. Zoom dies. Switch in Pygmy Cow
10. Chew
11. Udder Destruction
12. Chew
13. Feed

Snozz (Post-Shadowlands update)

After doing so many battles, wasting 100 bandages or more, old tactics are not working because of the recent nerfs, this one worked from the first try, and then it worked a few times afterwards with 0 losses.
Perhaps the RNGesus was with me because it had been 3 days straight of trying and failing. I hope it works for you too.

The MVP of this fight is the speed buff for all the 3 pets, having the upper hand with speed, the fight was very easy.

Lava Beetle H/S (preferable to use the one with health breed) 2,1,1
Darkmoon Rabbit H/S 1,1,2 (Any rabbit with the same abilities should do)(Must have more health than Squirrel)
Grizzly Squirrel S/S 2,1,2 (Any squirrel with the same abilities should do)

Lava Beetle
> Pass

Darkmoon Rabbit is forced in

>Vicious Streak
>Stampede
>Stampede continues (Your Darkmoon rabbit must survive the encounter, if it dies -very unlikely, restart the fight)

Rabbit is swapped for Squirrel (Rabbit should be alive)
>Adrenaline Rush
>Woodchipper
>Stampede (Your Squirrel must survive the encounter, if it dies to due to an extra flank hit or crit, restart the fight)

Void gate swaps in Beetle
>Hiss
>Scorched Earth

Beetle dies before Void Gate

Choose Squirrel
>Stampede

Void Gate kills Squirrel and your Rabbit enters
>Scratch

Victory.