Use HAMA agent with ASS scanner for serialised/Plex Video Files Scanner for movies (seperate library). Setup is kinda shitty (ie. not optimised for the average user), creator doesn't provide very good instructions either, so I wrote some out below:
Download the zip file and unzip to %LOCALAPPDATA%\Plex Media Server\Plug-ins. Remove the -master from the folder name.
Donwload this and unzip, and copy the contents of the unzipped folder to %LOCALAPPDATA%\Plex Media Server\Plug-In Support/Data/com.plexapp.agents.hama/DataItems. Create locale if needed.
Copy this text to this file location: %LOCALAPPDATA%\Plex Media Server\Scanners\Series\Absolute Series Scanner.py. You will need to create this file (and file path). Just create a .txt file and change the extention to .py.
Create Anime (TV) library using 'HAMA TV' agent and Abosulte Series Scanner. Create Anime (Movies) library using 'HAMA Movie' agent and Plex Video Files Scanner (or Plex Movie. Not sure which one works best tbh). Customise settings to your fancy.
Other metadata agents for anime include: AMSA and MALA .
I use MALA (which uses MAL to pull metadata) as a secondary agent for my movies library as AniDB fucks up with some movies (either it has trouble finding them or it lumps a number of films in a single series into one entry (e.g. Kizumonogatari)