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
Spend at least 60 seconds considering something futile
Solve Ps and Is
Use Hint 100 times
Solve As and Hs
Solve Fs and Us
Solve Cs and Us
Solve Ys and Rs
Solve your first Hard puzzle
Solve Es and Xs
Solve Ys and Es
Solve Us and Ps
Solve Ms and Hs
Solve Ds and Os
Solve Ts and Os
Solve Ls and Os
Solve Hs and Es
Solve Is and Ts
Solve Gs and Ns
Solve Bs and As
Solve Fs and Ms
Solve Bs and Gs
Solve Es and Gs
Solve Bs and Os
Solve Ts and As
Solve a puzzle without making a single mistake
Solve a puzzle in under 60 seconds
Solve Xs and Os
Solve a puzzle without using Fix or Hint
Solve your first Beginner puzzle
Read the instructions to the end