Same here, Melandroso. I kept Moonshell's shield and mists up and he lasted a little way into the final enemy pet. As you say, Crawdad just sat back and watched the show - didn't need him to enter the battle.
Worked very well with a P/P Moonshell Crab and found that, by weaving in Shell Shield and Renewing Mists, the crab was able to finish off Squirt and chip away a little bit at Squeezer's HP. Turtle and Hogs finished the mission admirably.
i figured that out also before reading your comment... great minds think alike i guess :-) (worked for me as well) tried as written a few times and it failed for me .. then decided to keep shell and mist active.. and never even needed the third pet :-)