We discuss several methods to realize efficient rescattering of the ionized electron in various relativistic regimes with the recollision energy spanning from the kilo-electron-volt to the mega-electron-volt range. In an energy domain of several tens of kilo-electron-volts, the recollisions are achieved via assisting the infrared strong laser field with a weak attosecond pulse train (APT) of extreme ultraviolet photons. For recollisions in the mega-electron-volt domain, strong counterpropagating APTs are employed. High-order harmonic emission in these setups are enhanced by several orders of magnitude in comparison to the case of a single laser wave. This way hard x-ray harmonics and extreme short pulses can be feasible.
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