why not "cudownym partnerem"?
Seems okay, added.
Why is it "jest wsp..."?
in the sentence "he is noun" you use on jest + noun (instrumental), if an adjective describes the noun it has to agree with it in case,number and gender.
Singular masculine instrumental of wspaniały - wspaniałym
I have a slight suspicion that Ellikin could have "why there are five consonants in a row?" in mind.
It wont take wyspanle now. Only cudowym
"wyspanle" doesn't really look like any word that I know. You mean "wspaniałym", probably - and that's the main answer.
Hovering over wonderful suggests świetny as a possible translation but it's not accepted in the sentence. Is it not appropriate for context?
You still need to use it in the proper case. So that would be "świetnym".
'On to wspaniały partner'?
"to wspaniały partner" is not accepted
Actually, we decided a long time ago that we don't like the 'to-construction' when used with pronouns. It may technically not be wrong, but it's quite rare and sounds rather clumsy.