Once you hit Tier 5 I found the quickest way to gear up early on is to farm the 15 named bosses. I’ve gotten full clears down to 30 min solo. You’ll even get an occasional exotic or classified if you are lucky. They re-spawn every 4 hrs.
I’ve numbered the map and added my own notes:
1. Autumn's hope - exit facing E, R, he's just to the L
a. bullet king
2. Wolves den - exit facing E, R, Down ladder, R, L @ school bus to cut through truck yard, R, basketball court on L
a. Sorretto - basketball court
b. Strings - train tracks, continue through basketball court heading S, R all the way to the end, L down ramp into train tracks
3. Kerman station
a. Erskine - exit facing E, turn 180 after exit from escalators to the left. Veer L to find crates to get over wall. Keep heading west to end of street. He's just a bit north where all the semi trucks are.
4. The Cavern
a. Fumes - underground station. Take N Exit, up first 2 ladders. Left before 3rd ladder, left again, just to right of subway
5. Hudson Refugee camp - start facing west, go L, L again into alley way
a. Fray - alley way
b. Five-o - , continue south to 24th, R, L over small fence, L into back alley way, continue E
6. Napalm production site
a. Belzer - start facing E, R to go S
b. Riot - run to union square, it's a long run, you can run past most mobs along the way.
7. The ward
a. Chains - exit facing N, immediate left, then left again. Head straight down covered walkway (it's a little to the R) and up stairs, go left past the bikes on your right, veer left
8. Police Academy - start facing E, turn 180 to go W, L, L, he's in a parking lot on the L
a. Cpl. Wright
9. DZ checkpoint E 31st St
a. Sgt. Lewis - just head straight E.
10. Queens tunnel - starts facing E so just head forward. Follow east 38st street all the way down East
a. Cpl. Dorsman - stay left to avoid mob at gate on the right. Just run past them.
11. Firewall - exist facing N - R, L to continue N.
a. Sgt Jacks - north, go right and up little stairs into outdoor patio area of restaurant. Engage from there.
b. Apollo - rope on right side, attack from behind signs above awning.
Once you get 4 piece d3fnc along with a pretty good smg clearing goes a lot faster. I like 2 piece nomad to go with it for the 15% heal on kill. Once you memorize all the spawns running sticky bomb with flashbang works well. You don’t even need a heal.