This can be done in any zone with relative ease. Stay as close to the top of the screen as possible to keep the level scrolling steadily, use the flamethrower as much as possible on ground forces to quickly cut through them, switching to your machine gun when your fuel runs out. Use nukes and grenades at the onset of boss fights to gain a quick advantage, and use the flak cannon or flamethrower of weakpoints to quickly end these fights. Also make sure not to go into lairs when going for this achievement. This achievement can be unlocked on any difficulty.