We present spectroscopic measurements in which utilized are line-intensity ratios in the near-uv to measure electron energies of several hundred eVs, which usually necessitates the use of emission in the soft x-ray region. The main intensity ratio selected is of the BIII transition 1s(2)2s2S(1/2)-1s(2)2p2P3/2 (2066A) and the BIV transition 1s2s3S(1)-1s2p3P2 (2822A). A detailed atomic-kinetics modeling is made to demonstrate the usefulness of this atomic system for studying transient, ionizing plasmas. Here, it is applied for the characterization of high-electron energies (approximately 500 eV) generated due to the rapid penetration of a magnetic field pulse into a low-collisionality plasma. Limitations of the use of the method are discussed.