Challenge 9 felt more challenging than level 10 due to Ilsix and his 30% resolve. This is one of the most difficult mechanics to deal with and the problem is further complicated by his high damage and random spawn of jammer, water, and heart orbs if you do manage to get him into the 30% “safe” zone.
One of the most important aspects of Blue Sonia is her naturally high base HP and incredible 6.25x RCV for dragon and physical type monsters. This allows her to be extremely durable in coop play where your HP pool is augmented and you are able to essentially use actives in quick succession in short floor dungeons. My dual colour row-cascade matching comes from a lot of practice in endless corridors and is an amazing way to improve your damage output and combo potential.
I also cleared Challenge 10 with Blue Sonia, but was unable to record and the two screenshots should provide some inspiration. I used my burst damage prior to Vishnu and was a matter of chipping him down without surpassing 2 million damage as he already halves water damage.
Best of luck in your coop challenge endeavours.