The easiest way to obtain this is to defeat all the monsters in any level (first is easiest) and reach the gem pile at the end. At the gem pile, keep hitting
to reload the letters on the screen until you have all the necessary letters to spell BACON.
During normal battles you can reload the letters at the cost of passing your turn and you take damage from the enemy, but at the gem pile there is no enemy so you are free to reload as often as you want.
This method works for the BANDITS achievement as well...
(edit: Thanks Rathelon
for the tip)