It's a playful and slightly ironic way to refer to someone who is:
* Extraordinary: Someone with amazing abilities or talents.
* Exaggeratedly good: Someone who is very good at something, perhaps to the point of being boastful or showy.
* Fantastical: Someone with a lively imagination and a penchant for the unusual.
The name "Fantastisch" is a play on the German word for "fantastic" which is "fantastisch." It's a common way to add a humorous tone to a name or title.