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