Pop and I went up Mission Peak on a beautiful January morning. We took the “normal” path up the Hidden Valley trail then cut across Hidden Valley on the Grove Trail to Horse Heaven Trail to get to the peak. That’s definitely the best way up and down. http://www.ebparks.org/files/Mission_Peak_map.pdf
We took Horse Heaven trail to the bottom, and the last 2/3 was a killer. It’s VERY steep after Hidden Valley.
Photos from the hike: