Decrypt Progressive Cipher Puzzle

I’m sitting on data (Text and .xls - other formats available) that hides a message in a video game. All previous digging suggests the text was encoded with a per-message progressive body cipher—possibly the GAK or XGAK variant—but I’ve hit a wall on how to pry it open... (Budget: $128 - $512 USD, Jobs: Algorithm, C Programming, C++ Programming, Cryptography, Data Analysis, Data Processing, Mathematics, Python)