I got this game recently, and followed everybodies advice of doing it in zone 1 to no avail. I tried going in the lair, not going in teh lair..etc..no dice. I even started a new profile, and went through all of zone 1 on foot, spent a good 5 minutes killing small spiders on the spider boss..no dice.
To make matters worse, I created a new local profile went through all of zone 1 on foot, killing everything I could, spending time on the spider boss, and racking up a total of over 600+ kills. Not only did I NOT get the achievement, I actually go the Peace Maker achievement (less than 50% of enemies killed in zone). That told me there was something screwy going on with zone 1. I suppose you can try the method to spend 15 or more minutes at the spider boss to rack up 1500 or more kills..but I have a better method.
I did the exact same thing in zone 2..and got the achievement first time. Kill everthing on foot except the zone boss..kill him in the car (or you'll die too fast). Achievement popped.
Hope that works for you.