In the sentence "I had had great experiences before, but this one was like a whole nother level", I understand the idea, but I'm also aware that saying a whole nother is not grammatical. Is there an alternative way of saying the same sentence such that 1) it's grammatically correct and 2) does not lose the original meaning and emphasis?
Instead of "a whole 'nother level," you could say the following: paramount, superior, revolutionary, unparalleled, or incomparable.
Of the words listed above, I think "unparalleled" best conveys the intended meaning of your sentence.
"I had had great experiences before, but this one was unparalleled."
If you want, you can qualify unparalleled to be more specific: unparalleled in its _______ .
"Whole 'nother" is often paired with "use ta could". Those who use these types of colloquialisms often have additional meaning than a more grammatically correct term. Any word/phrase you can use to get another human to understand you is good, spelling it with correct punctuation can indicate that is the way you intended, if not using the latin (sic). Dadda beda!