The Buff is a Superpower and the element you need for this achievement is Fire so you want to equip the Superpower-Buff and the Element-Fire.
The easiest and fastest way to get this is to go to the Alien controlled area in Steelport (South of New Colvin and East of downtown should also be white on the map)
Fully upgrade the heavy mini-gun (or a gun you are more comfortable with) and the Superpower-Buff. Activate Buff and kill enemies with the mini-gun dont kill enemies if you dont have that fire wall around you and dont get to close to aliens when you activate it because it wont count as a kill, and dont shoot cars because they do not count as kills if they kill aliens.
"There's a virus upload side quest in Pierce's quest line that has one wave of 101 enemies in Genki's suit to kill. I used this quest to do all my kills-related challenges. Just kill about 90+, leave, wait for mission to expire, and then restart mission.~(neeker75)
Hopes this helps pm me or comment if you have any questions or anything to add to the solution. If you give me a negative vote please comment or pm why so i can fix it.