For this achievement you need the "Sleep Darts" and the "Animal Taming" Skills in the Seer Skilltree. For taming a lion you will also earned an achievement.
In the territory Sapi-Res Nome is a Camp called Pyrrhos nearby a fast travel tower. In this camp I found every time 2-3 lions in cages. Clear the camp from the enemies and equip your sleeping darts on a lion in a cage. After the lion is sleeping, tame the lion. Now look with your eagle for a crocodile, preferably by the water. If you found a crocodile go to this location with your tamed lion and lure / bring the crocodile to your tamed lion. Both animals are fighting now.