Hammer a slightly smaller socket on. Probably an imperial and then as you try and undo it tap the bolt with a hammer. It breaks the seal. Mechanic showed me that trick on my motocross bike when i fucked my sumpbolt. He got it undone in a few minutes

Best thing with sump bolts or any bolt thats too tight and feels like its gonna round off is to tighten it slightly first. This breaks the seal as well