I assume that depends how her power works. IIRC it was never really described to be anti-life or the likes, but rather negative energy. The name of the power, and her showing when Michael and Tiffany were evolving, makes one think she drains someone's monster power.
If you look at her ability like that, she'd be positively useless against anyone with power relative to or higher than hers, in other words, Bernhardt, Henri, and Malik, possibly even some or all other level 5s.
It could even be argued that Ella was only affected so drastically and/or quickly by Nyx's move because Malik greatly tired her power reserves beforehand, as I remember someone (Malik I think?) saying Ella's invulnerability thing probably used a lot of energy, not to mention Ella practically went through a gauntlet beforehand. Otherwise, it could be argued that Ella just had lower power reserves compared to the average level 5 since she just recently evolved.
Then again, she does have abilities directly related to life (or death rather), such as resurrecting someone, so there may be more to it than that, but we can't quite tell.