Choose loadout as a crossbow with thermite arrows and everything for mobility. Leave the other slot free so you run faster when in danger.
Lethal: Throwing knife can 1tap normal zombies, 2tap armored.
Tactical: Decoy Grenade, so you can put all zombies together and the crossbow thermite will do the job.

Ideal perks to carry on or buy: 1- Speed Cola, 2- PHD Flopper, 3- Stamin-up

You just need to pack-a-punch the crossbow ONCE, for 5k gold (2 fast missions on tier 1: Eliminate HVT preferably)
Then go to tier 2 and you can 1tap every zombie, do 2 more missions there so you can buy perks.
Then go to Tier 3, buy all three perks and you can 1tap any tier 3 zombie, and 2-tap the special zombies.
Now just loot, you can do some infected strongholds for better items, or some missions.

If you find it hard, go to a stair with only 1-way up and just shoot the floor and zombies wont go through, so that way you can eliminate easily.