You don't need to enter the Dark Zone or play daily challenges. Just do this minute-long grind over and over.
You need Phoenix Credits to buy the high-end gear in The Division, right? You can go the legit route: enter the Dark Zone, or take on the daily and Challenge missions.
Or you could take advantage of this simple trick to farm 1-3 credits at a time. That doesn't sound like a lot, but it only takes around 60 seconds to do. You'll need to be level 30 before Phoenix Credits mean anything to you.
First, go to the Autumn's Hope safe house. Make sure you have the Pulse perk equipped.
Run out the door and turn right, then left. You'll see four Rikers gang members. Pulse them so you can see which one is the high-end yellow named boss.
Focus your fire and kill him then pick up his drops. You will get 1-3 Phoenix Credits each time. You also get purple gear/weapons and there's a chance he'll drop the yellow high-end gun Caduceus.
Let the rest of the gang kill you, respawn at the same safe house and repeat.
If he doesn't respawn it means you've either killed his whole group or you need to do the supply drop mission to the left of his position.