This is not truly my area but I do know some people who study it, the assumption that the wars were insignificant is incorrect, kingdoms the size of Zimbabwe and later the truly vast Kingdom of Mutapa are not formed by small wars or maintained by small or incompetent armies. The problem of course is very little record remains.

We do have some catalogue from Willem Janszoon Blaeu and Portuguese sailors whose ships wrecked on the coast of Lower Africa and who were then rescued, from this we can determine that: Hit and run warfare was common and often used but that there were cores of organised men who fought in something resembling a shield wall, certainly they used encirclement and distraction tactics, skirmishers and poisoned arrows are often recorded.

Tactics appear to be fairly similar to those used in Northern African Tribal nations, skirmishers advance and break up the enemy formation, using bows and thrown spears, try to sow disarray and confusion, sometimes over several days and then the main force of the army advances and breaks the exhausted and demoralised troops.

During the Portuguese trading with the Kingdom of Mutapa the Portuguese were said to be much impressed by the army of the Mutapans, the Portuguese launched several attempts to seize said Kingdom by force (most notably "The accidental Crusade") but all were defeated though often this was as much due to disease as warfare. We also know that both the Kingdom of Zimbabwe which later became the Kingdom of Mutapa were able to fend off attacks from Arabian slavers and could raise armies of thousands and campaign many nations away.

I know this is very rough and will hopefully be replaced by an expert in the area but maybe a start.