From the Super Mario Wiki, the Mario encyclopedia
Lethal Slinkers are a rare variety of Slinkers appearing in ScareScraper mode in Luigi's Mansion 3, added in version 1.3.1. They are red and black, glowing Slinkers that do almost twice as much damage than their normal counterparts.. They have 300 HP, more than double that of a regular Slinker. They are defeated the same way as normal Slinkers.