You can achieve a Crushing Victory in every PvE, Boss, Raiding Brigand (Loot Wagon), Brigand Island, and Stronghold battle. Winning a fight with a Crushing Victory will unlock a gold bonus reward in addition to other victory rewards for this battle.
The only requirement to obtain a Crushing Victory is to win a battle within a certain amount of turns. Each battle has its own individual Crushing Victory requirement. At the beginning of every turn, a blue banner will notify you of the amount of turns remaining to achieve a Crushing Victory.
Even if you don't meet the Crushing Victory condition, don't worry, you will still be able to collect the normal victory rewards!