Here is a ROM hack which changes the regular game into a boss rush mode.
Mega Man 6 (USA).nes - NOINTRO
CRC32: 1EFCAC48
MD5: 37BB1A39A69BC6D1BCCAE0DD5411330D
SHA-1: 32774F6A0982534272679AC424C4191F1BE5F689
SHA-256: 86E069792ADDF21322355C245A0B155BA74E123B4A3EB9CD347175444569D122