he is in gruncle stan's brain and gruncle stan is mind controlled

that's how gruncle stan remembers everything bill searched through for important facts

bill can also, do pretty much anything im sure he can turn gruncle stan into his minion

he pretended they beat him when really he was stronger he is just waiting to catch them by surprise

he will return and mable and dipper wont beat him this time

if gruncle stan really survived that, he would have been better quicker in the time it took for "gruncle stan" bill can say a spell