Well, its not quite true that the Zulu's simply rushed directly at the British lines.

In fact, Zulu tactics were relatively sophisticated, with the entire society based around the development of age-based regiments of men, each with distinctive insignia. These regiments would be organised into a formation said to resemble buffalo horns, where the horns would be used as feints, or for flanking and encirclement, the head would comprise the schwerpunkt of the attack, and the loins would be kept in reserve to exploit opportunities.

It was this ability to move and co-ordinate large bodies of men that enabled the Zulus to engage better armed European forces at battles like Isandlwana successfully. The Zulus were able to leverage superiority in numbers and tactical sophistication to offset their disadvantage in firepower.

In fact, in engagements where British troops were caught in the open, without some form of fortification, the Zulus were extremely dangerous. But put the soldier with his rifle in a trench or behind some sand-bags, and you will pay a very high price to extract him. A foretaste of things to come....

Guerrilla tactics were actually what the British expected, and the period immediately before Isandlwana was spent chasing small numbers of Zulu to little effect. Chelmsford saw no harm in dividing his force before the battle as he simply never envisaged the Zulu's being able or willing to stage a coherent attack on that scale.

Such tactics would have been quite useless however. Standard practice at the time was to use a scorched earth policy, with cattle being confiscated, and crops and homesteads burnt down. This would have been quickly fatal to a pastoral society like the Zulu nation.

The Zulus did have a limited number of firearms. These were mainly obtained through trade with various Europeans, however the firearms in question tended to be dated - Brown Bess muskets, for example, and training in marksmanship was poor to non-existent, meaning the assegai and shield remained the primary weapons.

Sources:

Brave Men's Blood - Ian Knight David Rattray - Oral history