There is no such thing as an impossible DC since a natural 20 succeeds all the time. Setting an "impossible" DC just means you can only win on a natural 20.
But note that this is a just in the Larian adaptation. Per normal 5E ruiles there is no critical success or failure on skill checks, unlike for attack rolls.
This being said you can make DC 30 rolls with decent probability, though you have to build towards it. Some example:
+4 Proficiency + 5 Stat + 1d4 Guidance + 1d10 Bardic inspiration
Alternately:
+4 Proficiency + 5 Stat + 4 Expertise + 1d4 Guidance.
That is doable with luck, and not unlikely if you can also get Advantage.
You could combine both Expertise and Bardic inspiration to be even better - but admittedly that is very special party composition.