Here is the full list of all 31 Tic-Tac-Letters by POWGI trophies - there are 4 bronze, 23 silver, 3 gold and 1 platinum.
Collect all the trophies
Solve Ys and Es
Solve Es and Xs
Solve Ys and Rs
Solve Fs and Us
Solve Ps and Is
Solve As and Hs
Solve Ts and Os
Solve Ls and Os
Solve Us and Ps
Solve Cs and Us
Solve Ds and Os
Solve Ms and Hs
Solve Gs and Ns
Solve Hs and Es
Solve Bs and As
Solve Es and Gs
Solve Is and Ts
Solve Bs and Os
Solve Fs and Ms
Solve Bs and Gs
Solve your first Hard puzzle
Spend at least 60 seconds considering something futile
Use Hint 100 times
Solve Ts and As
Solve a puzzle in under 60 seconds
Solve Xs and Os
Solve your first Beginner puzzle
Solve a puzzle without making a single mistake
Solve a puzzle without using Fix or Hint
Read the instructions to the end