pokemon shadows of alma

I'm stuck in the ruins fighting this plant monster called Spiritomb he's a grass/poison i think how do i defeat him what's he weak against? It's been years since I played pokemon before so I forget who's weak against what.


I know water is weak against eletric grass weak against fire but that's all I remember

He has no weakness...>.>
Just most attributes don't affect him.

Hit it hard with a fire-type, it generally deals more damage then all the other types, by just a little tho'