1. Take a high recoil gun (sawed off).
2. Point it down.
3. Jump and fire.
4. Now flank on them poor people.
5. (Optional) With the heavy barrel mod you can increase recoil causing a high jump.
Note: The desert eagle when duel wielded with heavy barrel works for this as well.
Note2: You can use the megagun (a very rare minigun varient) for this however its technique is just using it as a jetpack instead of as a second jump .