Blizzard has announced nerfs to the Mythic difficulty of Nymrissa Wavecaller, focusing on tuning down the AoE damage happening during 
Abyssal Rain.
Nymrissa has been an incredibly challenging fight on Mythic in its current form. Despite having dozens of public logs attempting the fight, no group has succeeded in defeating her. The major challenge of the fight has been the incredibly high healing check it poses, with some groups going as far as 6 or 7 healing it to no avail.
These changes will likely make the fight much more approachable, especially the nerf to 
Frost Burst, which was dealing over 200k per explosion on Mythic.
Nymrissa Wavecaller Tuning Changes
Hello,
We just sent a hotfix with the following changes to Nymrissa Wavecaller on Mythic difficulty:
- Abyssal Rain’s initial damage reduced by 12.5% on Mythic difficulty
- Abyssal Rain’s periodic damage reduced by 20% on Mythic difficulty
- Reduced Abyssal Rain’s damage scaling based on group size for larger groups
- Frost Burst damage reduced by 40%