This tactic is called a hit-and-run.
Actually, this wasn't a hit-and-run by definition. In a hit-and-run, you harass your opponent and then retreat from the battlefield (run from English).
Furthermore, it was possible to win with the Orcs (slowing down the enemy with Shamans, controlling units, or consuming corpses, and other minor tactics). However, as already noted, the Orcs would still suffer significant losses, unlike the Necropolis.
