I found it really hard too. And my Slice ship is my best ship. I can do 5th missions really easily, even on auto with more than 50% life at the end. But 6-5 is too hard. I failed twice on it with all girls max level, max affection and full of event equipement almost max level + 4 rainbows max ampie/2nd affection and Oncidium greatly ampied and 2nd affection too.
This 10% instead of 40% is really painful. This and the lack of bloomed girls available.

So... let's farm the good old neutral whaleship maps.