I think your Nemu cant do more than 50% dmg. Or her health must be lower than 50% of team health. Cant remember which one even though I did it earlier today
Have your formation like this : Only Main and shy nemu, no zanpakuto, do it asap when shy is still low level, put no equipement on her so she takes damage and it should be fine ( condition is health below 50%)As a side note you can and you should do all of her bonds with this formation