Defeat 666 Enemies While in Havoc Form.

This one may be the last achievement you get along with "BFA" as it is a total grindfest. To enter Havoc mode, you must press 'LB' + 'RT' as the spiral red icon begins to deplete. As there are not that many areas with many enemies, I found it best to serpent hole to Haven - West End. Run around to the far end and into the garage levels. Then go into the garage by the Stasis wall. Here, just corner yourself as all the enemies gather and you unleash Havoc mode. Equip an upgraded Obscurus to a weapon for the Havoc meter could fill faster. Also, press 'LB' + 'RT' again once you've defeated the group of enemies. This will save you on the Havoc meter. Since the game doesn't keep track on how many more kills you need, the achievement tracker does - you get 7% (or roughly 46 kills) this way.

The only downside is that you must die for this area to respawn with enemies again. Buy shards of Havoc from Vulgrim to speed up this process.