Hold X while choosing an attack. Keep the button held until you attack to do more damage.
To get this secret ATAC, successfully complete the game once. Then, in Mission 10 or 11 you will have a multiple choice answer during your conversation with the princess: ''I intend to end the war'' and ''I will conquer the continent''. If you choose to end the war, you will go to mission 12. If you choose to conquer the continent, you will advance to mission 45 ''Ruin Branch'' and also get TIC-TAC.