On the docks in Apollonia in Krenaika (Recommended level 34-38), the northwestern most part of the map, there are a bunch of animals in cages including a lion and crocodile. After clearing out a couple guards, you can tame a lion and then open the cage with the crocodile and they will start fighting getting you the achievement. The guards in this specific area are level 37-38 and the animals 34-38. This guide is for later game play as the enemies are higher level, but is the only spot I know of that will guarantee both animals being together.
Note: You must be the same level or higher to tame animals.
A visual aid for the location:
If you divide the dock into two sections, the crocodile will always be found in the largest cage in the southwestern section with a lion usually in the cage directly next to it to the right. The animals in the smaller cages may not always be full or have the same animals, but there will always be multiple caged lions nearby.