Problem 12 (complex): Roots of Unity are commonly found in problems! They aren’t necessarily scaled by 1.
Problem 19 (number theory): Answer choices can help to eliminate four possibilities, leaving you with the answer.
Problem 21 (number theory): Modular arithmetic isn’t really that complicated, it can be used to solve a variety of problems.
Problem 22 (combinatorics): Instead of immediately brute-forcing cases for counting problems, instead consider cases of what we are looking for. This could narrow down our work.
Problem 23 (algebra): Just be more careful when doing algebra. Also plan your general process in your head before starting to write.
Problem 24 (geometry): Not all geometry problems require a complicated or unique idea to solve, sometimes they just require calculation.
Problem 25 (number theory / algebra): When a problem seems to be confusing, always try with smaller cases first to see a general idea.