We experimentally demonstrate the amplification of optical-field-ionization soft x-ray lasers in an optically preformed plasma waveguide for pure xenon, krypton, and argon gases, respectively. The lasing photon number of Ni-like Kr laser at 32.8 nm generated in waveguide is dramatically enhanced by about three orders of magnitude in comparison to that without plasma waveguide, resulting in a photon number of ~5×1011 and an energy conversion efficiency of ~5×10-6 with a pump pulse of ~500 mJ. By seeding optical-field-ionization plasma with high harmonic signals, 32.8-nm Kr laser produces brighter x-ray laser beams with much smaller divergence, enhanced spatial coherence, and controlled polarization. Single-shot x-ray digital holographic microscopy with an adjustable field of view and magnification is demonstrated successfully with the illumination of high-brightness 32.8-nm x-ray laser. The ultrashort x-ray pulse duration combined with single-shot capability offers great advantage for flash imaging of delicate samples.