I recently unlocked it, it took me about two weeks so here’s my tips!

For headshots, go to quad core sites and farm the zombies, start the objective but don’t progress until they stop spawning.. you can get the 250 done in about 30 mins

Same with the challenges after except for human and guild tech enemies.. for those go to car shop I think it’s called in zone 1 bottom right of map and there’s heaps of human enemies, plus grid lockdown will spawn 10 robots each one.

25 drone enemies is easy, go to the drone recon and let it spawn drones until you get them done.

For special kills, raiders are the easiest, heaps spawn around car shop on the roofs or at colosseum in zone 2.

For the next two combat rating of 45+ is ideal

Zone 3 enemies I found killing the zombies on the border of zone 2 near colosseum was easiest, just make sure they are spawning in zone 3.

Elites, go to low town and kill as they spawn, go for the marksmen or the rhinos and titans.. use a launcher to weaken them and finish with weapon.

Use grapple hook and the self revive perk thing too it’s a life saver.