Does the mule at least have access to Tavnazia? If not, take some time to get it. The vultures just outside of town are, I believe, 83-84. With Signet up, you'll have slight defensive and evasive benefits. Given the recent changes, even the level 83 ones can get you to cap.

They'll still hit kind of hard, though, and you'll not want to keep your RDM behind the MNK since they have some conal WS moves, but between Counters, cures, Signet, and MNK's defensive stats, you should be able to manage.

Part of me wants to say to just tough it out to 90 and then come back to this, but I can understand being a bit of a completionist.

As for actual Atmas, Regen ones can be found from Fistule (Kon), Lacovie (Tahrongi), Kutharei (Mis), Resheph (Ule), Adamantoise/Aspid (Synthetic), Grau's zone quest boss (Though it's daytime regen only), and two more synthetics from completing Bastok missions and all 50 assaults.

Ones I personally used for this purpose were the first three, as that granted a constant 50/tic Regen on top of other defensive boosts. This is kind of diminished since you're limited to one active atma, though. Strive to get more when you can.

I can't speak for MNK's DD atma choices since my own isn't leveled, but at the very least, you will want to get Razed Ruins from the Ironclad in Mis.