The puzzle in this game is surprisingly hard. I love it XD
I beat all of the levels and enjoyed every levels XD
Sadly, the tutorial is hard to understand. Maybe you can add number in the white circle, so the player can understand better what shift and reverse do. I got stuck long in the early levels because of that. You can also add "drop letter" in the tutorial. I got surprised the first time it happened to me.
The level design on the early levels are also good. It forces you to take/drop some letters and then using shift/reverse to rearrange the letters.
Sadly, the later levels didn't require that because the player can just dodge any unnecessary letters or just reset every time it happened.