Well there's multiple things you can do.
Firstly, you don't want to stay at the same level, muscles get used to the exercises but also for example if you fail at lvl 12 with Typewriter pull ups, you don't just want to keep trying it till you get it because you wont get it, simple as that:
-Redo lvl 9,10,11
-When you get to level 12 again, you can use a resistance band to assist yourself if you have one, or do a progression of the specific exercise (in this case the typewriters)
At least that's what I did.
Or if you *reduce* levels (or in other words: go a few levels back), before your actual level/workout you can practice the exercise you're failing/you can't do yet.
Pistol squats needs strength but also balance, so get your balance in check and do a lot of pistol squat negatives, the basic strength ones - lunges, squats, wall sit, and assisted pistol squats by holding on to a chair or something. For balance - squats with feet together, and just sitting on the bottom position of the squat (with feet together) and trying to sit on one leg on the bottom position. Good luck!