As this problem is quite lengthy, with some quite challenging codes towards the end, it could be offered as an extension for those students who have sailed through the easier codebreaking problems.
One possible approach for the whole class is to divide the group up into teams so that each team can tackle the challenges collaboratively, in competition with the others.
If a computer room is available, introduce students to the Cipher Challenge Toolkit and give them plenty of time to explore it and learn how to use the tools.
The Stage 5 Cipher Challenge offers a set of seven very difficult linked ciphers to crack.
The challenges have been carefully chosen to get progressively more difficult; the first three should be accessible to anyone who has completed Substitution Cipher.