Just as the description says, kill 400 Haunted. I wouldn't worry about this achievement. You will get it naturally if you are going for 100% because you will likely be playing 4-5 playthroughs.
You will have your normal playthrough to learn the ropes.
A 2nd playthrough to finish upgrading Sebastian and gather all the collectibles
A 3rd playthrough to beat in under 5 hours.
A 4th playthrough to beat it on Akumu(DO NOT try to stack achievements with this one, it isnt worth it. Just makes it more ridiculously harder than it is. Took me 40 mins alone to beat chapter 2, not counting all the times I died.)
Lastly a 5th to beat the game with no upgrades.
At that point you will likely have all the achievements done.