Named enemies are dropping between 1 and 3 Phoenix Credits. They had them at like 10 to 15 for a few days, but people abused that and farmed to cap easily in the DZ in a short amount of time. Now the easy way to farm them is to do Challenge mode.

Tunnel seems the easiest, probably followed by event center / Russian place (depends on who you get matched with), while the plant is a sh!t show with the 8 min timer to clear 3 packs of mobs with a random group (timer starts at 8:45ish if you wipe after clearing the first group of mobs that's already there when you look for the pipes to close or such) - not too hard with a group of smart friends.

Overall, do the daily hard modes as they are quick and give you an easy 16 to 18 Phoenix Credits. The daily challenge will net you 51 to 53 Phoenix Credits in varying time, depending on which place it is that day.