A firm supporting grip of the components and small wiggles, would be my summary of the usual technique to break small components like buffers free of the bufferbeam. Using a small blade, scrape off any visible adhesive before starting. The adhesive Bachmann use is pretty brittle, and usually parts cleanly with a snap.
But isn't the bufferbeam integral to the running plate in this model, and thus to be discarded? In which case you can cut into the bufferbeam around the buffer, and break it away piecemeal.
Make sure the major parts are an easy fit and go fully 'home' before committing to the reassembly. The one 'not quite right' body I have had from Bachmann was an A1 with a slightly curved (cast metal) running plate. Cause was a tiny piece of flash on the casting which made the plastic firebox side tight to the casting, and led to the slight deformation. Easily rectified with a wipe of a file once disassembled.