Single word for something that becomes progressively harder to succeed at

That game offers diminishing returns with each successive spin.

The boxer became more evasive towards the end of the fight which led to diminishing returns for the fans who stayed up late hoping to see some action.

diminishing returns
1. In economics, a yield rate (i.e., of profits, production, benefits, etc.) that fails to grow in proportion to the amount of investment, skill, time, or effort that is added. The restaurant, to combat high volumes of customers, hired a large surplus of wait staff and cooks. This led to diminishing returns, however, as the overcrowded staff was far less efficient and eventually cost the restaurant more in wages than it was earning.
2. By extension, any output or results (e.g., of a product, project, organization, etc.) that fail to increase proportionally to additional time, money, skill, or effort. Unfortunately, the show's charm has not lasted, and the infusion of zanier plots has created diminishing returns in terms of quality.
— Farlex Dictionary of Idioms

'Facing Exponential-Odds' comes to mind.

While defining both "exponential" and "odds" would take a whole pile of words, I do think people are familiar enough with the concept. They understand it when a sports announcer says something like "with every moment of time that elapses the odds of overcoming the score deficit become exponentially higher"

the Dictionary.com, third definition of Exponential

3) rising or expanding at a steady and usually rapid rate: a city experiencing exponential growth.