Before attempting this, I recommend you to max out Knives to level 16. This will make it a lot easier.
The difficulty doesn't matter here. You simply need to beat ALL 7 levels from start to finisih in one single session without quitting. Which will take you about an hour at level 16.
To save a little bit of time, I recommend to use the subspace Highways segments in the levels 1, 4 and 5 for shortcuts.