The Mirror-slicer Array for Astronomical Transients (MAAT) is an IFU now being manufactured for the OSIRIS spectrograph on the 10.4-m GRANTECAN (GTC) telescope at La Palma. It is based on the Advanced Image Slicer (AIS) concept as are MUSE and KMOS on the VLT (among many others). Very significant changes to the design had to be made due in part to cost limitations but mostly weight limitations. We present the final design, its expected performances, and the process to get there, in particular the difficulties encountered to avoid variable vignetting in the spectrograph due to the telescope derotator and the primary hexagonal shape. This especially includes modifications of the pupil shape and a new method of extreme optimization of AIS that has been 25 years in the making and that push the Zemax software to its extreme limit. MAAT will take advantage of the OSIRIS mask cartridge for multi-object spectroscopy. The IFU will be in a box that will take the place of a few masks. The field is 10" x 7" with 23 slices 0.305" wide giving a spaxel size of 0.254" x 0.305". The wavelength range is 360 nm to 1000 nm. The spectral resolution will be 1.6 times larger than with a standard slit of 0.6" because of the smaller size of the slices. The eleven gratings will deliver R = 600 to 4100. In an effort to maximize the resolution of a spectrograph designed for a slit twice the width of the slices, we measured the wavefront of the spectrograph by using 2 out-of-focus masks with pinholes along the slit and compared it to the as-designed wavefront. The goal was to modify the IFU pupil mirrors to partly cancel the aberrations in the spectrograph. This showed a residual wavefront variable over the detector due to tolerances, even negligible in some parts; wavefront almost only made of focus, coma, astigmatism and spherical aberration.