There are merely 4 steps to reach the ultimate function within the same round

1. Don't be afraid of Change. But embrace it instead.
2. Keep on changing your mind whenever the COPER asks you to pick element
3. Find a Level 9 Squad, and stick to it (if leveled up, leave and find constant 9)
4. Talk to the OAK

If it didn't work, Buy Item#332 and Item#192, go to PROF Si and USE them there.
Try again.