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