You know, it occurs to me, there's a prominent American landmark they've been avoiding up to now..... (Disclaimer: Not based on anything except the inside of my head.)
Sam rubbed his face. "Okay, if you can't just wish Lucifer out of existence, how can you help us?"
God smirked slightly. "I can help you acquire the one weapon that can make you credible threat to him. Would you like that?"
Dean straightened. "You know where the Colt is?"
"Mr Colt's work, while quite impressive, isn't quite up to this job." Reaching under the diner counter, God pulled out what looked like a rolled-up blueprint. "You need the work of the man who continued his work." He unrolled the paper with a flourish. "You need the Winchester ."
Dean and Sam stared at the diagram on the paper, a repeating rifle with some very strange enhancements. After a minute, Sam shook his head.
"Some of this we can cannibalize from the Colt, but..."
"The rest would take much to long to create from scratch, yes," God finished. "Fortunately, most of what you need is already made and waiting at your ancestral estate."
"Our what ?" Dean gaped. We don't-"
Sam broke in, "Wait....you mean the Mystery House?"
God made a tick mark in the air. "Right first time."
"Okay, whoa, wait." Dean waved his hands. "Dad told us never to go to the Mystery House, that if there was a curse, we'd probably set it off."
"Well," God shrugged, "sometimes even if father knows best, you have to disobey anyway." He paused. "Consider the source on that one, too."