This is a cumulative achievement and can be done over many levels.This is not in a single Fury mode but over as many as it takes.
Fury mode is activated by pressing B when the Red gauge in the top left corner of the HUD is over half full (Marked by an arrow)
From here all you need to do is kill 496 enemies which is not difficult and is not effected by difficulty.
This is exactly the same as it's previous achievement, just requires 9.92x the kills: