Most of my time has been spent in career mode so Im not 100% on this, but it seemed to easy to get a sub switch for me. Going after the acheivement for sub switch and getting a sub vs the cpu in under 60 seconds took
one attempt for me.
I used BJ Penn vs someone with low sub D. I dont even remember if I just grabbed him and pulled guard or did a quick takedown and slapped on the sub from full fuard. Either way I let him waste energy for a max of 3 seconds while I did NOT spin the stick, did the sub switch, and let him waste energy for another second or 2 and then started to shine and it was over. I didnt go back and try vs higher level guys but it was still easier than I expected.