This is a hard one. Level scaling means the enemies in a video game scales with the player. What's the antonym of that term? I would prefer it if it was one word and it didn't contain the word scaling.
I'm not exactly sure, but linear gameplay is a term you might find interesting. To be perfectly honest, "linear gameplay" is a concept that's actually the opposite of another similar concept known as nonlinear gameplay which is a gameplay mode where you have a certain degree of freedom in deciding what path you're going to take playing the game. In simpler words, the scenario of how the game is going to develop has not been predetermined for you by the game's developers. The following is an excerpt from Wikipedia elaborating more on these two ideas:
A video game with nonlinear gameplay presents players with challenges that can be completed in a number of different sequences. Each player may take on (or even encounter) only some of the challenges possible, and the same challenges may be played in a different order. Conversely, a video game with linear gameplay will confront a player with a fixed sequence of challenges: every player faces every challenge and has to overcome them in the same order.
Although the term "liner gameplay" is probably not exactly the term you're looking for (it's not even certain if there is such a term), it does fit your requirements because the enemies and challenges that you're confronted with in a video game with a liner gameplay do not actually scale at all: it's the exact same thing every time you play the game. And isn't that what you're asking?