Follow these steps;
1. Load up hex editor (e.g., HxD).
2. Open save game in hex editor.
3. Search for your Bros name.
4. Slightly before the first mention of your Bros name (likely in his description) there will be a text “Human” (not in the hex values, obviously).
5. Keep moving right from the hex values corresponding to “human” (the hex editor will highlight both in text and hex), and find the “09” hex value following that.
6. After the “09” you will have eight hex values, spaced with a “00”. These are your attributes. Change them all to whatever you want (e.g., 9F to make them all 159).
7. Save. Load the game up. You now have killer starter bros.