The generation of attosecond pulses through the process of high harmonic generation has opened the field of attosecond science where attosecond phenomena are directly investigated. At present, attosecond pulses are produced through up-conversion of IR/visible light to extreme ultraviolet. We propose to amplify the photon-energy of attosecond pulses from the extreme ultraviolet (30 - 100 eV) to soft x-rays (100 - 1000 eV), through a wave-mixing process between the extreme ultraviolet attosecond pulse and a mid-IR field, in pre-ionized gas. The mid-IR field is too weak for ionizing the ions and generating high-order harmonics, independently. However, high harmonic generation is initiated through single-photon ionization of the extreme ultraviolet attosecond pulse. Subsequently, the ionized electrons are accelerated by the mid-IR field to very high kinetic energy (which is proportional to the wavelength square of the driving laser) and finally recombine with their parent ions, emitting an isolated burst of x-rays. The fact that ionization and acceleration of the active electrons are driven by different fields facilitates the generation of isolated attosecond pulse at very short wavelengths.