Dissonance of Nexus DLC
First, turn on the new Increased Affinity Gain setting at the Main Menu screen, selecting Options and then LB to get to the far right tab for Other Settings where you can change your difficulty level (easy/normal/extreme) and now you can have Increased Affinity Gain (on/off). (This is a new setting from Title Update 1.6.0)
The method below will work on any friend once they are on your Friend List and you can add them to your party. This method can also be used for any of the 9 or so base game achievements related to gaining affinity to make affinity gain a LOT easier/faster once you have access to the DLC story from the New Game option at the Main Menu, and selecting your base game save.
I noticed on NG+ and after beating all DLC storylines, once fast travel is available during the Tutorial, I could go to White Frontier. However, this doesn't seem to be the case for a brand new save file with a level 1 character since I hadn't beaten the DOTN DLC on that save yet (if anyone can confirm, and I will also continue and see when the DOTN DLC is first made available during a base game playthrough unless someone can update me sooner).
Yuna, Eiji, and Seven will be added to your Friend List as you progress through the DOTN DLC main story line. (not all at once, but incrementally)
Each of them have 3 sleepover scenes, but you only need to do the first one. Using the method below, I maxed out their Affinity to level 4 with 100% before I looked for their chat bubbles (green chat icon) in the Urban district in SBC Glocken. Choosing the top/positive option generally will be the correct choice in order to have the sleepover. Credit to Armoured Brecce for confirming that you will probably need to have friend affinity level 4 before first being able to talk to them.
I unfortunately started to grind the affinity out the hard way using compliments and trying to get the AI to suicide on the monsters so I could get revives.
Ironically, I found a very simple and easy way to gain affinity after I was pretty much already done. (tested this on friends other than Yuna, Eiji, and Seven)
Method: Since the White Frontier now has infinitely respawnable bosses. Head to the White Frontier starting Fast Travel point and head north/northeast. Waste Gang will be located in this upper corner of the map. You may have to blast through a group of scorpions/bees to get to him, but he is one of the simpler bosses to kill in the new Field. After killing Waste Gang ONE time, and once you get out of combat, fast travel back to SBC Glocken.
The single/first boss kill in White Frontier nets you affinity (on Extreme, one Waste Gang kill/fast travel):
level 1: +70%
level 2: +70%
level 3: +50%
level 4: +35%
These numbers might be around 150-200% of the base affinity gain with the new Increased Affinity Gain setting turned on.
So, killing/traveling about 8 times will get a Friend's affinity from 0 to max level 4, 100%
These numbers are for ONE kill and then fast traveling, NOT multiple kills and then fast traveling, you will need to fast travel in between each kill. You can stick around and kill him multiple times but it won't be scalar multiples of the numbers above. (Ex: You can kill him multiple times and find you only get 90% of an affinity level)
As always let me know if anything appears to be wrong, or how the guide can be improved.