I believe you'd need to use ICARUS scripting to set the level to change once an NPC with a specific NPC ID is killed. I can't help much beyond that, sorry, but my ICARUS skills aren't as high as they should be.
To create ICARUS scripts, you'll need to use BehavEd, which is packaged with the SDK.
You may be able to link the NPC to a trigger brush set as target_level_change, but I'm not sure. I'll try that later and if it works I'll post up a quick tutorial (provided no one beats me to it).