  One of the main science goal of the future European Extremely Large Telescope
will be to understand the mass assembly process in galaxies as a function of
cosmic time. To this aim, a multi-object, AO-assisted integral field
spectrograph will be required to map the physical and chemical properties of
very distant galaxies. In this paper, we examine the ability of such an
instrument to obtain spatially resolved spectroscopy of a large sample of
massive (0.1<Mstellar<5e11Mo) galaxies at 2<z<6, selected from future large
area optical-near IR surveys. We produced a set of about one thousand numerical
simulations of 3D observations using reasonable assumptions about the site,
telescope, and instrument, and about the physics of distant galaxies. These
data-cubes were analysed as real data to produce realistic kinematic
measurements of very distant galaxies. We then studied how sensible the
scientific goals are to the observational (i.e., site-, telescope-, and
instrument-related) and physical (i.e., galaxy-related) parameters. We
specifically investigated the impact of AO performance on the science goal. We
did not identify any breaking points with respect to the parameters (e.g., the
telescope diameter), with the exception of the telescope thermal background,
which strongly limits the performance in the highest (z>5) redshift bin. We
find that a survey of Ngal galaxies that fulfil the range of science goals can
be achieved with a ~90 nights program on the E-ELT, provided a multiplex
capability M Ngal/8.
