If you have been stingy in your use of bombs, go into training mode and set your bombs to 10. Increase your lives if you so desire. Select one of the bosses and just bomb away to destroy the boss through the use of bombs. Rinse and repeat. Best to do it in Hard Mode. The bosses are more resilient, requiring more bombs, and you are protected because of the constant barrage of bombs. The trophy unlocks once the total number of bombs dropped in all play modes combined reaches 290.