Easy "Love Ending Unlocked!" achievement
Save the game in stage 4. Choose the "Grab his hand tightly" option, then complete the game to get the love ending. Reload the saved game in order to unlock the other ending.
Easy "Normal Ending Unlocked!" achievement
Save the game in stage 4. Choose the "Take his arm normally" option, then complete the game to get the love ending. Reload the saved game in order to unlock the other ending.