Noo, ONLY 15. The boss isn’t class restricted….
Yes and no. It can be done with as little as 6r3, so they don't need to all be "high" rank, but there are a couple things working against us.
First, there is no free healing in-between fights. So the lower rank champ you use, the more chip damage you take, and as a result the more items you will likely have to use.
Second, the nodes on all but the mutant chapter are incredibly limited to a fairly small percentage of each class. This means it is difficult to go in with only your top champs because some of them will just take a long time to get through fights.
The design still needs some work to become something really good. Unfortunately, you're going to need at least 90 different champs (15 of a class per chapter, plus a champ for each boss fight) and the content is more difficult than it needs to be because of the combination of different minor issues that need to be corrected.