As this problem is quite challenging, it could be offered as an extension for those students who have sailed through the easier codebreaking problems.
If a whole class is going to work on the problem, they could work in small groups and share out responsibility for checking different transposition arrangements for each likely substitution.
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.
Spend plenty of time working on and discussing the separate Substitution and Transposition problems before trying this one.