This achievement can be earned EARLIEST in the first world "Jibberish Jungle" on the last level "Hi-Ho Moskito!" (Though the final level of each world allows you to ride around on Moskito.)
While flying around press X to inhale enemies, after a certain amount you'll need to fire them with A. Also you don't need to inhale all 50 enemies in 1 go, so take your time on the other levels. But inhale 50 enemies and you'll net this 15G achievement.