TM80(Rock Slide) is the north-western corner of Mistralton Cave on the 2nd Floor.
The Rock Slide TM has an odd location that's tough to explain, but yes it's in the top-left corner of 2F down a narrow pathway.
This tutorial has step-by-step screenshots to guide you the whole way.
And here's a final screenshot of the TM's location, taken from the tutorial linked above.