Equip the Ultimate Booster I and II system chips in your motherboard to get this achievement more easily. This will boost your Flux ability, most importantly increasing its duration so you can mow down more enemies in a shorter time.
There are two very easy rooms filled with basic enemies for this achievement :
- Licking the Wounds has a room near the beginning with 8 melee enemies and 2 hammers.
- Mindgames' second loop will spawn a bunch of melee flunkies right before the last corridor. You can't miss it, the door locks until you kill everyone.