My toughest challenge of all the tablets was the dead center one. I found the last one surprisingly feasible with a bit of luck, but otherwise with good reflexes they were all achievable. My biggest challenge overall was cutting enough time to be able to have 4-5 seconds times on the circles, because i'm not that good at pattern recognition. Since you're able to get under the required time for all the other challenges, you're able to be bad at the things you're bad at and focus on the rest, which i really liked.