Azazyel
You have definitely written yourself in a corner here. This would have been reasonable if it was another normal space marine chapter coming here but the grey knights. They are literally the best trained for this type of scenario with the most discipline and techniques and technology and more powerful psykers and anti -psyking technology. Like literally even if he was a few cultivation levels more he wouldn't be able to deal with them and then even if for some force majeure he defeats kills or most likely escapes then they most likely will know exactly how to take you down next. BRUH