While the Murloc and Pirate synergy is no doubt useful in those compositions, Cookie value in the current meta is most felt via the Appetizers equipment, and his third ability unlocked at level 15, Go Fish!
Just like in competitive Pokémon, winning Mercenaries PvP is largely influenced by two things: a player understanding of ability speeds and whether or not their lineup has sufficient synergy to out-value the opponent each turn.
