We have carried out a concept study for a wide-field monolithic integral field unit (IFU) spectrograph for extremely large telescopes (ELTs). We target in this paper the technological challenges that have to be faced in order to build such an instrument, focusing on the adaptive optics (AO) requirements, the image slicer technology, and the detectors status. We also address the main science drivers, together with the concept design and the expected performance applied to the European-ELT (E-ELT) case. A monolithic wide-field spectrograph provides a continuous field of view (FOV) separated by a field splitter in several subfields, each of them feeding a module featuring an image slicer, a collimator and a spectrograph. The use of image slicers provides 3D spectrographic images of the complete FOV, allowing for detection and study of sources without need of targeting them, a very useful property especially for the deep observation of faint high-redshift objects, whose density on the sky is expected to be quite high. In light of this discussion, we suggest the advantages of using shorter wavelengths and its implication in both the scientific program and the budget.