Ultima HM is really frustrating

[Judge_Xero]

Biggest thing is probably to designate a party healer and a tank healer. Often both healers will be using AOEs to top the party up while the tanks drop off.
Also for the Bit phases if I happen to be OT at that time I've used AOE threat to grab the focus of the majority of the bits which helped the healers a lot. I'm not sure if you have to be the first to generate hate on them or if you just need to be the highest. But when you do just run East away from the party and dodge lasers.

[PenutButter]

This fight is definitely one of the most challenging for healers, especially if you're other healer blows. Fortunately, the fight itself is not that hard. Trying to heal the tank while dodging things like Ifrit can be stressful, which can be alleviated by good tanks. SCH has much easier time with this than WHM due to Lustrate and, to a lesser extent, the pet.

[kukurumei]

This fight is definitely one of the most challenging for healers, especially if you're other healer blows. Fortunately, the fight itself is not that hard. Trying to heal the tank while dodging things like Ifrit can be stressful, which can be alleviated by good tanks. SCH has much easier time with this than WHM due to Lustrate and, to a lesser extent, the pet.

Sch can do fancy worth with lustre, but succor is even better in ultima.

There is a good amount of RNG even with i90 people, but it's mitigated by good teamwork. There are only really 2 times a tank can die. Outer bits(lasers) and Bombs 2 (from unlucky tank purge)

Healers don't have to be good so much as the rest of the pt needs to be good. Ifrit need to be dodged if it's coming from behind the tank. Bits (especially outer) need to be downed fast.

Pld's stoneskinning each other usually helps a lot in certain phases. As with dps that can focus and kill bits quickly, as with Melees that don't get floating bombs hit.

Healing is actually quite lax, if you simply save your big stuff for the trouble spots, but nothing will work if the other members are derping.

Ifrit dash
erruption
lasers
bombs

Those are the normal hurdles. and most of them can be mitigated by good teamwork.