Hold Z + R + L when the level description appears at the start of a level. Note: The invincibility only lasts for the current level.
Go to the ant hill from the main screen. Then, hold C-Up and press R at the "Flick's Journey" screen. An arrow pointing to the right will appear at the bottom of the screen to confirm correct code entry. Press Analog-stick Right to scroll the screen in that direction to access any level. Alternately, hold C-Up + C-Down + C-Left + C-Right + Z and press R.
Return to training level and find the letters to spell "FLIK" to gain a bonus life. Repeat this to obtain as many lives as needed before resuming the regular game.