Start up a genocide run on Hard Mode
The encounters are a bit harder hitting but otherwise the same
Then you go to replay it again
The monsters are still battle scarred from your last run
They still remember what you did
New attacks and tricks

Here's an idea: Have Mettaton NEO be just as weak as he is normally, in return for him taking the initiative and making you endure a furious last stand. He'll have a feeling he can't hold back and try and eliminate you before you get the opportunity.