The question becomes one of uniqueness.
If the words are being used as a descriptive then you can not
preclude others from using them
ie. ______ is a "fast car."
Your goal initially should be to find out who this is.
Do they have deep pockets
Did they file this letter because they believe to not send
a demand letter would be to waive their right to the name.
Does the state of minisota use the phrase "minisota fishing"?
How many other sites use the phrase minisota fishing.
The more normal and non-unique you can make the phrase the
weaker their claim is.
One last point, they may have a lawyer who convinced them
they could hijack the phrase by sending out the letters
Respond, and identify how you are not diminishing their claim.
hope this helps.
ps: is it just the phrase or is there a logo involved?