The Impi can repel any pre-Medieval mounted unit with great ease, due to being as fast as, if not faster than them. This makes the Zulus safe from quick attacks.
The Impi is also a very effective scout unit - able to move quickly into position and look for any oncoming armies, giving the Zulus time to use the Aggressive trait and Ikhanda to form a good defense.
Finally, the Impi can be used for pillaging purposes - being effective against Mounted units, your opponent will have to send a slower retaliation - giving time to escape.
While Impi have a major speed advantage, they're not any more effective against Axemen and can be defeated quickly by them. Stacking Axemen and Spearmen will serve as counters to any quick units the Zulus may send at you if you're attacking them.
The Zulu spearmen were warriors without peer in southern Africa, dispatching every force that they encountered until running up against the superior technology of the Europeans. The Zulus were organized for the first time by Shaka, who began forming spearmen into groups called "impi" even before he became the chief of his tribe. Impi warriors began training as early as the age of six, serving first as porters and servants, and when of age, the cadets were trained in the use of weapons. When Zulu males reached adulthood at age twenty, they would become full members of their warband.
Zulu spearmen were expected to be able to outrun a horse, travel 50 miles a day on foot, and move silently through rough terrain. The difficult training that the Zulu warriors went through made them tougher and stronger than their opponents, who frequently outnumbered the Zulus in battle.
When going into battle, the Zulu spearmen painted their faces with chalk and red ochre to make themselves more intimidating. They carried spears (called "ilkwa") made of wood with a long iron point. The ilkwa was a stabbing weapon, unlike its predecessor the javelin-like "assegai," which had been used for throwing. Along with the spear, Zulus would carry a medium-sized shield. As contact with Europeans became more frequent later in the nineteenth century, Zulu warriors also made use of muskets and rifles captured from the British, although the Zulus did not have the training to use them properly.
Tactically, the soldiers of a Zulu impi often used Shaka's "bull horns" formation, which consisted a main force ("isifuba" or chest) and two wings on the sides ("izimpondo" or horns) designed to outflank the enemy. A fourth group of veterans would remain as a reserve in the rear.
The Zulu spearmen formed an effective and highly-versatile army which proved to be almost invincible when facing other African forces. They traveled lightly and could move very fast (if not the 50 miles a day that they claimed), but they also packed enough force to disrupt heavy enemy formations. Shaka used hit-and-run techniques against the numerically-superior Ndwandwe in the 1820s and achieved complete victory in a series of engagements.
However, the Zulu spearmen were no match for the Martini-Henry rifles and Maxim guns of the British army when they went to war in 1878. The technological rift could not be overcome with Zulu valor.