I used Songhai because they start as a monarchy instead of tribal. Also, there is a Tripoli core on one of your starting provinces, meaning you can recruit Muslim troops, including cavalry. Played at Normal difficulty.
Fighting fellow Africans is ridiculously easy by the way. Battles last forever, so while half of your army is fighting the enemy army, you can just walk over their entire unfortified country with the rest.
So I united the West-African countries, took Colonial Ventures as my first idea and colonized the two provinces going through the Sahara, Tuat and something, around 1470. There I got a border with the Mamluks and Algiers. Befriended the first, conquered the latter, waited and westernized. After that it wasn't particularly difficult.
Built some naval bases to raise my naval force limit, so I could take some islands I needed from Great Britain. Got the achievement around 1700.
Edit: 4000th post! Yeah!