Age of Empires II: Definitive Edition (PC) - Achievement - With the Help of My Sisters
Amass an army of at least 100 Gbeto Warriors in the fourth Sundjata mission "Blood on the River Bank".
The first thing to note in this mission is that you do not initially have access to Gbeto Warriors, which you need for the achievement. You can only build them from the Castle after you complete a mission for Mema (Green). I believe the mission triggers when you first build a Market and there is no time limit to complete it so take your time.
The mission requires that you rescue their princess who is being held captive in a walled outpost containing an enemy Castle just to the West of Mema. You can get to it by going through Mema's settlement and crossing the shallow water. There are a few towers, Pikemen and Trebuchets, but nothing too difficult once you have some Trebuchets of your own and a light force to defend them with. Once the Castle and towers are destroyed, the princess will come under your control. Bring her back to Mema's Castle and you will be rewarded with 15 Gbeto Warriors and the ability to train new Gbeto Warrios from your Castles.
It should be noted that the achievement does NOT trigger simply by training 100 Gbeto Warriors, but you must have all 100 of them alive at the same time, which can be a little tedious considering you have a population limit of 125. Just make sure your economy is strong early on and get rid of a bunch of your villagers when you need the population. Also, Elite Gbeto Warriors still contribute towards this achievement - and so do the 15 that you given as a reward for rescuing the Mema princess.
The only real risk to be aware of is that if you complete the missions for both Mema and Fakoli (Yellow), they will both become your allies and are capable of killing your main enemy, Sumanguru (Red) pretty much on their own so be careful that the level doesn't end too early before you have trained the warriors you need.