If you are on your second playthrough and you still haven't gotten this achievement, you can obtain it easily in the final dungeon.
On your way through the final levels, you will encounter an armored boar. As per the story, you will need to take her down once. After that, she continues to regenerate, regardless of how often you take her life down to 0. A green mist exudes from her while she's regenerating.
Instead of running, arm yourself with a Very Light one-handed sword (damage really doesn't matter) and hack away at her without hesitation. You'll be able to easily knock her into submission and continue your chain while she's regenerating. You should reach a 100-chain without even trying.
I specify a second playthrough simply because she's pretty effin' tough on the first.
Achievement should pop right after the chain count breaks 100.