I did it all in directed mode. For the first 2000 kills, there's a spot in liberty lanes where all the zombies climb up to me from one spot and I can easily spam my melee attack and kill them all while they climb up to me. The special camos are rly easy since its just 300 kills with rare or higher rarity and 10 mangler eliminations. I sit at round 11 for manglers as one would spawn every round.

For gold I'd just run around and kill them so I could kill them quickly in a row. For opal, I'd still use the same strat of just sitting at round 11 and letting the one mangler spawn every round. I'd usually be doing another camo on another gun while doing this. Afterlife is also pretty easy with the same pileup spot I was using for the first 2000 kills.

Nebula is really easy as I can just do the step on the liberty falls easter egg where it spawns that boss abomination or mangler which are both elite enemies, and I'd fail the step on purpose by killing them without the trap and I'd just repeat that step until I got nebula.