Step 1: Drop the difficulty down to Explorer and dismiss your sidekick (they steal kills).
Step 2: Load up the Five Stars Theme Park on III Difficulty (
Goggs25 confirmed I Difficulty works as well,
Siphon Filther was unable to get it to unlock on II Difficulty in Feb of 2019 but
rIKmAN unlocked it in April of 2020).
Step 3: Go straight for the package and make sure you are seen so reinforcements are called.
Step 4: Grab the package and follow the waypoint to the extraction area.
Step 5: Get on top of the ticket booth and pull out your binoculars and tag enemies as they approach. Jump around rooftops to gather them together and keep them aggro'd. When you get to 11 enemies you're ready.
Step 6: Drop down to ground level, activate wrath by holding down

, then
quickly press

in when you're near an enemy to use Eden's Wrath. If Eden's Wrath doesn't work, mash

instead until they go down or bounce away. Reinforcements will show up on quads so you'll have plenty of targets if you want to avoid the Elite and shielded enemies.
EDIT: In the comments,
ChaosPanda BB notes that you can use pipe bombs and proximity mines and the kills still count.
Imothep Czech confirms this made the achievement easy and he got it on his first try and
AgentOfAbyss states that only the proximity mines worked for him.
EDIT July 2019: My video solution disappeared from my Xbox drive. No idea what happened. I got 15 downvotes in 1 week because the video was broken. My apologies. Here's a similar video to help you out: