A very easy spot for this achievement early in the Solo campaign is 1st Mission-Dead Ahead, 2nd Chapter - Locomotive Lockdown, Difficulty- Medium or Easy, Enemy setup- 4 players.
Character- Jun ( for speed ) Not Necessary though, any character will do
Weopons- Mosin Nagant, Trench Gun, C96 Pistol (weopons I used )
Or your best upgraded Rifle and secondary
Perks that will help but not necessary- Combo Extension, Combo boost, Heavy
4 Frag Grenades from the spawn in area
153,000 score needed for Gold
Now go up the stairs at the beginning of the chapter to the door with the yellow star, but before opening go to the turret behind the door and detach it if you selected Heavy as a perk. Then drop it behind the ammo crate right behind where you detached the turret.
Now open the door and walk about half way down the ramp to trigger the cut scene. After cut scene toss a frag at the zombies right in front to get your combo meter going, then back up to the top of the ramp. Shoot the zombies jumping off the train letting some come up the ramp towards you( watch for the one behind to your right coming up the stairs) Once a few are close enough to the top of the ramp turn around with a full combo meter and sprint back behind the ammo crate where you dropped the turret and quickly start sniping the zombies at the top of the ramp to not lose your combo.
Now snipe them as they funnel through the door making sure not to miss and lose your combo ( shoot for the body). Now your set up and the rest is easy, just take your time and keep an eye on your combo meter. Some zombies wiill drop down to your left so be ready to switch to your secondary or turret if you detached earlier. You only need around a 115-120 combo to reach over 153,000 score, then just finish chapter. Restart checkpoint if you miss or get hit and lose your combo.
If you want to keep your combo going even further to say a 300-500 it's actually very easy to chain every area in this chapter together. Just leave a few zombies to shoot right before you sprint to the next area, giving yourself enough time on you combo meter to start shooting the zombies in the next area. If you miss or get hit and lose your combo just reload checkpoint to start back with your combo. I was able to reach a 497 combo, with 2,343,730 score on this chapter using this method.