Fighter Flies are located in Stardust Fields and their other form the Super Fly are located in the Bean Bean Castle sewers. In Mario Bros. Fighter Flies look a lot different than the Fighter Flies on Mario and Luigi: Superstar Saga also they are located in the sewers.
Fighter flies have one main attack which is charging at Mario and Luigi. To counter the attack watch how the Fighter Fly moves then bounce three times on the third bounce make Mario and Luigi jump to deal damage to them they have a HP of 4.