Route 28 is your best bet now, by a considerable margin. If not, though Cerulean Cave 2F is certainly no paragon of accessibility, it is still probably somewhat preferable to Seafoam B4F in that regard, and it actually gives a little bit more experience at night.
So Route 28 and Cerulean Cave 2F seem like the best places for a Level 42 Pokemon. After it grows a few levels, try Mt. Silver and rematching the Elite Four again.
Source
Hope I helped!
