Some questions -any spirit bonding tips for matching ilevel to mob level?

I've got some leftover gear (i145-150) from the HW story quests and since I can't sell it I might as well fully spirit bond them for materia.
I've tried to find some tips online in where to find farming locations in terms of ilevel and mob level, but I can't seem to find anything that works. I was in some ARR areas and the level 50 mobs didn't do anything to increase the percentage so I'm guessing they are too low. The level 59 mobs in HW was the same thing, so I'm thinking they are too high? Is there a formula based on gear ilevel to know which mobs to farm?
Also, is there any decent ways to get elemental materia, especially tier 4?

Any advice/info would be much appreciated. ;D

It might take a few kills in Azys Lla for that i145-i150 gear to SB a point. Failing that, try ARF or Neverreap/Fractal Continuum.