Context.Stellar flares affect all atmospheric layers from the photosphere over chromosphere and transition region up into the corona. Simultaneous observations in different spectral bands allow to obtain a comprehensive picture of the environmental conditions and the physical processes going on during different phases of the flare. Aims.We investigate the properties of the coronal plasma during a giant flare on the active M dwarf CN Leo observed simultaneously with the UVES spectrograph at the VLT and XMM-Newton. Methods. From the X-ray data, we analyze the temporal evolution of the coronal temperature and emission measure, and investigate variations in electron density and coronal abundances during the flare. Optical Fe xiii line emission traces the cooler quiescent corona. Results. Although of rather short duration (exponential decay time tLC <5 min), the X-ray flux at flare peak exceeds the quiescent level by a factor of ∼100. The electron density averaged over the whole flare is greater than 5 × 1011 cm-3. The flare plasma shows an enhancement of iron by a factor of ∼2 during the rise and peak phase of the flare. We derive a size of