I will be honest mate, 20% difference is a lot. Even if you counter him, his mid sounds very strong. He will most likely be going through the middle so the only chance you have is to try 4-5-1V defensive and go through flanks on a counter attack (chances are you will be low on possession, and you will be playing defensive with defence bonus anyway).
If he counters 4-5-1V he will still be hitting mid unless he decides to match your formation (which is what I would do vs a weaker opponent).
But hey, this happened before... in a final a 18% weaker team smashed me 2-1.
Ensure you are 100% rested and 100% motivated so at least that's not issue.
Good luck