The recent adaption to Ruin/Ruin II did help in regards to leveling pre-50. The 30 ACN quest is the one, where you stand in the buff field with your ally. And as you don't need to dot single mobs up for max Ruin damage, that bit helped a bit in order to not make this quest more annoying by getting yourself or the NPC too mobbed up. Personally, after one very early terrible experience with Wind Carbuncle (where I had suddenly 5+ mobs at once clinging to me xD) with another character, I just skipped using it with Altina and waited directly for the yellow version. Their damage didn't feel too helpful either way to me.
The first time, the 50 SMN quest boss was a huge pita and I died quite fast. Thanks to google, I dotted enemy 1 up and ran around while spamming Ruin 2 and Energy Drain/Fester and managed to survive somehow better until the easier 2nd half of the fight started; with mostly the 45 job armor w/o chest part I think.

As for tanking pet and Earthen Armor: I only know the latter, but thought it was quite useful as it basically negated all damage as long as it was up. Again that's for pre-50 open world/job quest mobs. However as someone fond of actual pet classes in other MMOs, I'd have liked to play with the early carbuncle/egis to see it myself.