hia!
ok ive done it before and it might help u too:
go to the unit editor and select the unit u want as the flamethrower.
edit its attack range to almost 0 (40-50 will do i think), edit its effect art to be a flamethrower doodad which is found in spawned doodad or however it is called :roll: and by triggers do that if a unit is attacked by the flamethrower then pick all enemy units in range XXX to attacked unit, paste a visual effect on then and reduce their health.
WALLA!!
hope ive helped
GOOD NIGHT!!