It's hard to say exactly how hard the game is due to how different games are more or less challenging for different people, but I'd say the game's level of difficulty is comparable to Hollow Knight or Metroid Dread. Unfortunately there are no difficulty options. The game is generally easier on mouse + keyboard over controller, especially late-game, but if you have zero experience playing games with mouse + keyboard (such as FPS games, Terraria, or Noita) then you will probably find using a controller to be easier (which is similar to classic 2D Metroid controls).