Minibosses
In my opinion the hardest part of the dungeon are two of the minibosses: 211261 and 211263. The difficulty comes from the overlap of the high-damage, single target 451119 overlapping with unavoidable aoe damage from the minibosses' other ability, 633004 for 211261 and 450854 for 211263
The overlap is basically identical but because of how 633004 and 450854 work it will behave differently for each one of them. In both cases you will get a 451119 five seconds before the aoe ability is cast, but for 211261 633004 is a channel with several ticks, which means you will have the whole group taking damage over the duration while one person is taking extra damage from 451119 at the same time.
On the opposite, 450854 for 211263 is a longer channel that doesn't do damage during the channel nor when it finishes, but after a variable amount of time that depends on how much open space is there behind the target he cast it on. This will always overlap with a 451119 simply because there is almost always one debuff up in the group but assuming people position correctly, the orb will blow up five seconds after the application of 451119. This is a single high damage hit so unlike 633004 you don't need healing during it, you instead need to make sure this single target is at full hp before this explosion.
Handling these overlaps is key to dealing with these minibosses as the damage ramps up with key levels, and differentiating between debuffs that overlap with aoe damage and those that don't is key to not overcommiting cooldowns when they aren't needed and running dry before the hardest points actually come.
211089
In my opinion the worse dungeon boss in the season, not necessarily because its the hardest but just because its spell sequence is very convoluted and results in some overlaps that you can't really deduce intuitively
The cast sequence of a three minute fight might look something like this:
It is very chaotic, and has some nasty overlaps sprinkled in like either 426860 blowing up or 224616 getting 452099 casts off right as 426787 is gonna start doing heavy aoe damage.
Your main focus should be preparing heavy cooldowns for every 426787 and trying to heal the damage from 426860 while using as little resources as possible, so you don't overcommit things to it that you might need later.