You can cheese with Kiris or just keep the mushroom health low so that its exploding doesn't do much dmg and focus on Ken.

This stage can be cheesed with Dizzy, if you don't have her you will need someone with spd down and stun to prevent the mushroom from taking a turn. Bring F. Kluri for the def break and any soulweaver with cleanse like A. Momo because ML Ken can inflict blind and provoke. If you have Tamarrine and Iseria, that's better. The last spot goes to your DPS of choice, I have gone for C. Lorina. Or you can always go for the poison cheese strategy with Kiris / S. Tene.

Also, stack up souls during the mob fight if needed.