With yt-dlp you can download entire playlists and search results. This command downloads over 3000 videos that are probably related to キルミーベイベー in some way! Being logged into niconico and using cookies-from-browser is needed to get age restricted videos, and doubling every % is needed for .cmd/.bat files in Windows.

yt-dlp --cookies-from-browser "firefox:%%APPDATA%%\librewolf\Profiles" --download-archive archive.txt -i -o "C:\Downloads\nicovideo\%%(title)s %%(id)s %%(uploader)s.%%(ext)s" https://www.nicovideo.jp/search/%%E3%%82%%AD%%E3%%83%%AB%%E3%%83%%9F%%E3%%83%%BC%%E3%%83%%99%%E3%%82%%A4%%E3%%83%%99%%E3%%83%%BC?sort=f