The Approach
Keep pushing from city to city without pausing to fully rebuild between conquests — each capture upgrades the capturing unit outright (Corps becomes Army, standard becomes Corps), so an aggressive, continuous campaign actually strengthens your army faster than a slower, more cautious one would. Garrison Impi in newly-taken cities specifically, since the Loyalty bonus from a garrisoned unit is higher for Zulu than the baseline, helping captured territory stabilize faster.