Once you have the Storm Hollow go to the Catacombs and use it to hover across the previously inaccessible lava pool (Its the area with the wind and grapple spots). Here you will find the Unfocused Enoch enhancement. Equipping this automatically fills the havoc meter. At default it will take 33 minutes but with it fully maxed out it will take just under 6 minutes to fill up.
The Obscurus enhancement (Located in Forgotten Lake near the red glowing skeleton with the club) also generates more havoc when attached. However you need to physically use the weapon to generate it, while Enoch generates over time as long as its attached to a weapon. Put Obscurus on your whip and level it up to increase your havoc meter even faster.
I found a midgame setup to help demostrate a good location to grind in the video below
Sorry, this Game Clip has been removed
The South End has unlimited enemy spawns, which you can use to help fill up your meter faster. Once you get Havoc filled, teleport to the West End and you will find a large cluster of weak enemies.
While the South End is helpful for early grinding, I have since discovered that you can skip this step altogether once you have Obscurus and Enoch fully maxed (also I had arcane at level 20) Simply teleport to West End and proceed from this step like you would normally as seen in the video.
One critical tip to follow is to cancel your havoc form before the meter depletes. This can save over 50% of your meter with each grouping of enemies. The maxed enhancements fill up the meter so quickly you can have the meter filled in less than a minute as long as you only use up what you need.
The last thing to note is you need to die in order to get the enemies to respawn. This will be made extremely easy once the incoming patch adds the kill Fury option in the menu. But until that patch goes live on consoles, use the nearby cliff to drain your health and then find any surviving enemy to finish you off.
With this setup I got the achievement unlocked in about 45 mins