Is the camera static and do you have any clean frames of the background? You might be able to pull a difference matte and use that for the brunt attack, but it will take rotoscoping... a lot of rotoscoping.
Keyers like Keylight don't do well with luma keys, they're designed to work from the differences in saturation in each color channel. Primatte is similar, but it uses a weighted color matrix.
Angelo Lorenzo
Fallen Empire - Digital Production Services