How to do it:

once immerseus goes in to split phase and spawn sha puddles you need to cc this puddles for the entire phase,
once the split phase is over, the remaining sha puddles you have cc'd will transform in to Tears of the Vale they have very little HP and does not need to be tanked.

Note: you do not have to kill all 10 of them in the same phase, we did it in 2 split phases.

Few findings from my run:

The sha puddles are immune to Ring of Frost, Banish, Fear, and any other similar form of cc. They can be stunned, slowed, rooted and knocked back.

It may be wise to unequip legendary cloak, as single proc off on add can melt it.

After transformation, the new adds MUST be nuked ASAP. They are casting raid-wide aoe that can quickly overwhelm healers even on flex difficulty.
Boss still have enrage timer. You may not notice it on standard kills, but taking too many prolonged phases can end in wipe. Aim for 3-4 or 5 adds per phase.