There are 24 Zombie Challenges in Dead Island 2 (found by going into the  menu and then scrolling down to Challenges > Zombie Challenges) and in order to unlock 'On Safari,' you need to unlock the first tier of all 24 (and only the first tier, so fear not). There are 5 Challenge achievements, and of all the challenge trophies, this is perhaps the most straightforward.

To complete the first tier of every Zombie Challenge, all you basically need to do is kill a certain amount of zombies of all varieties. Check out our Dead Island 2 Zompedia guide if you're looking for more assistance on where certain zombies can be found.

Perhaps the trickiest part of the Zombie Challenges is killing 10 Inferno Crushers and 10 Firestorm Slobbers, but just head to "Hotspots" - as indicated on the map by a zombie icon - and you'll be fine. Beverly Hills has a Mutator spawn, while southern Venice Beach has both a good Slobber spawn (for Putrefied and Firestorm Slobbers) and a Crusher spawn near one another, and that spot usually spawns an Inferno Crusher too. If you're looking for Butchers, then head to The Pier!

Once you've completed the first tier of all 24 challenges, this achievement will pop.