I had to go to the Plaguelands to get an HH Blighthawk (had 2, wrong type, and none on AH--hint, hint) and to Silverpine Forest to get the right Squirrel (same situation as above. Seriously, there's money to be made here. =D ) Once I had them, got it in one try. whew. Tried soooo many other strats, but as everyone says, the Underwater RNG gods are devilish. Nice strat, thank you.
This fight is so RNG that I spent 2 hours trying and didn't win once before giving up after using all of my remaining pet battle bandages. Not worth the effort, and this strategy relies on them never critting.
I did this with Blightbreath, Bligthhawk, Ghostly skull. Toxic fumes until first pet dies, stun 2nd aquatic pet so 3rd pet (beast) comes in, and get 1 more toxic fumes off. Kill that beast pet with ghostly skull (slash) and get ghostly strike off on undead turn. Last fight is Blighthawk vs. his aquatic, with consume corpse + slicing wind.
This worked for me on the first attempt but I had to modify it somewhat. I don't have a ghostly skull at level 25 so I used an HH Unborn Val'kyr (11*) instead in the 3rd spot.
Bruce did not open with Bash. My blighthawk's first lift-off got bashed, however, so I just passed and tried again - the steps caught up with the guide at that point.
When my Val'kyr came out, I cast Siphon Life once and then just Shadow Slashed until Maws Jr. died. I died during my undead phase so if the fight had lasted longer this would not have worked, so YMMV. At least it worked though. ¯_(ツ)_/¯
Blighthawk: the lift-off never killed Bruce. Not even once. If your strategy relies on this much RNG on the side of the user, then it's no good. In the ten tries I did I did not once get to the last pet.
You most likely used a certain breed in your strategy, but did not mention it here. You should definitely rework the strategy (and test it again, several times, with your own pets).