JCoxeye's Guide to Fighting Mettaton NEO

Press the FIGHT button and attack him.

...Okay, fine, there's slightly more to say regarding Mettaton NEO than that.

A common misconception is that it's possible to not do enough damage to him, failing the Genocide/No Mercy route.

This is not possible, as Murder Level 15 The Murder Level is an internal variable that keeps track of progress through the No Mercy route. causes him to take upwards of 900,000 Damage Specifically, at Murder Level 15, he's coded to take 900,000 to 999,999 damage.

At Murder Level 14 or below, he instead takes 34,000 to 37,289 damage.
when hit, regardless of accuracy.

The only time fighting Mettaton NEO will fail to progress the No Mercy route is when you haven't exhausted Hotland and the CORE's Kill Count, The 'Kill Count' is another internal variable that keeps track of how many monsters you've killed in an area. haven't killed Muffet, or haven't killed the Royal Guards.

Also, if you CHECK him, you'll get some unique flavor text, both from the CHECK itself '* METTATON NEO - 90 ATK 9 DEF
* Dr. Alphys's greatest invention.'
and from the new battle flavor text. '* Stage lights are blaring.'