This can actually be done in one play through. There is plenty of barbarians around and as you kill them, more will spawn. as long as there is some unoccupied (shaded) hex, barbarians will spawn.
if you adopt the Honor social policy you will both get a notification when this happens in explored space. so simply go out exploring to reveal all corners of the map and these will keep popping up and you will be running around wack a mole barbarians and end up getting a 100 in one game. if you want you can select the Raging barbarian advanced map setting to get it faster, but you will get there even without it.