So for those who missed out of Jormungand, you'll really want to at least get Yatagarasu. She has an ATK↓ debuff and even has a 1 turn shorter cooldown than Jorms. Stat wise, her atk is pretty average and she has rather low HP, same as Crom, who are both our current lowest HP event eidolons. She's only 30% fire at max though so those who have Crom, it is unlikely she will replace her.
We'll have to wait til the event for actual experience on Ragnarok but supposedly her overdrives seem to consist of healing preventing for 5 turns if regular overdrive or strong defensive buff on itself if raging. Depending on how hard her raging hits, it sounds like the optimal choice would be raging overdrive since you can just def buff/heal through it and dispel or wait the timer off the defensive buff.