the Druid class will legitimately carry any run to completion on its own. Just pop your party skill and use your main attack non stop
Kate is nice to use to get an extra use of your party skill otherwise characters with passive skills are nice since you have no reason to use anyone else. Even Roxy who will make a harder final boss won't penalize you because druid is so overpowered you can 1 or 2 turn most enemies with having a surplus shield and incredible healing