I do recall seeing some boss in DMM wiki that has no rage bar and hence can't be forced into stuns though. Unless Google Translate messed up again.
My goal in the super duper long term (i.e. when-Thunder-has-a-Healer-SSR-released that scale of long), 6 SSR (4 main + 2 for reserve) x 6 elements =36. Also 6 new eidolons for each element that + 100% elemental damage. But I don't think the question was for me xD.