"掷币赌胜负" can be translated into English as "flipping a coin to decide the winner/loser (in a bet or game)" or more concisely as "coin toss to settle a bet".
Here are some variations depending on the context:
For a general bet or game: "flipping a coin to determine the outcome of a bet"
For a more casual setting: "flipping a coin to see who wins/loses"
The most straightforward and commonly used expression would likely be "coin toss to decide the winner" or simply "coin toss for the bet".