All of the enemy skill materias.

Maybe an experiment - in hojo's lab

Maybe helping newbies - beginner's hall

Maybe with the Forgotten - forgotten capital

Maybe with the forgetful - chocobo sage's house

After that last guess and hint, it feels like stealing, but I didn't even come up with the answer--it was my sister, and she came up with it without hearing any of the guesses or the last hint. She only heard the riddle and the fact that it was a non-plotline-essential materia (she basically got the answer from line 4).