“On a level” or “in a level” express different ideas.

In a game, in a level means, that while playing the game, you go from stage to stage, usually advancing to a higher degrees of difficulty as you go. By writing “In One Level” as a title, you can say “In One Level”. Each level in a game is considered a level and you are in the level itself.

On a level is more a ranking, maybe expression your experince, your number of wins, or your mastery of some skill. Some of the role-playing games have over 100 levels, which have nothing to do with the challenges you are faced, but more with how long or often you play. Does “in a level” applies to both writings?

“At a level” would be a neutral expression and can be used in any case.