Use a sledge hammer and a nail punch
I actually did this one night when I was a teenager, desperate to get the chain off a frame, and it was much harder than I thought it would be. I had the frame and chain lying on a block of wood, smashing the **** out of it for ages before the outer plates came off the pin
But yeah, get a chain breaker
What brand of chain is it, and how many gears are on the back wheel?