Atma of the Einherjar is a poor choice for Sobek, it's simply not reliable enough to be useful. You can also dodge Tyrant Tusk by moving to the side when it readies it (you need good reflexes for this.) I duo'd it with a Black Mage for most of my skins and found that if I stopped meleeing at around 35%, the Black Mage could finish it off with a volley of nukes (utilizing Enmity Douse.) So it almost never used Tyrant Tusk. I eventually switched over to Apoc on that too because in the rare situation that I was killed by Tyrant Tusk I could quickly reraise > Mijin > reraise to erase my weakness.

For tanking NMs, always RR + Gnarled Horn. The +50 AGI caps dAGI used for mob TP calculations, gives you +20% critical hit rate, and +25 evasion. Apoc can be good, depending on the NM, although sometimes I use others for a variety of reasons (unwanted quick magic procs on Ichi is really annoying.) Cloak & Dagger is useful when you want evasion & aren't completely capped on accuracy while Siren's Shadow is probably better when you don't need that accuracy.