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What I'd recommend is to put Liora in your team and play "Shields Up" on her so she tanks the direct hits. You should try to kill her minions as well, as they're powering up her ult. If you can keep it, then don't use the special stage ability before her ult - it's aoe and can easily clear the minions she summons.

Also, in patch 0.63.4 Nyrissa has been nerfed. So if you're on any earlier patch, please, download the most up-to-date version of the game and the fight will be easier!

I hope this helps!