To access Classic Co-Op, go to: Play
, and create your family team. To unlock the "Very Hard" difficulty, you must beat Easy, Medium, and Hard first.
To help ensure you win, here is a spreadsheet that the TA community has helped build:https://docs.google.com/spreadsheets/d/1CuqnF4ehkOvUsdKwMeh5...
Please note that this guide isn't quite 100% complete, but most questions are on here.