SWORD ART ONLINE: FATAL BULLET

UPDATE 1: A alternative way to raise affinity level has been found, I will leave the original solution down below as well. This was posted by Ridley Prime so credit goes to them and the Youtuber who posted the video.

This method will work but having it on the Extreme difficulty is needed to require some snipers to one shot a team member.
This goes for the affinity of any character you need to max out for the achievements, apparently the quickest way to raise affinity is to order them to standby somewhere in combat, where they can keep getting sniper or killed by something powerful without fighting back, and revive them. Revives increase the affinity by 5% or so each time. Here's a video on the quickest way found thus far which I'll take advantage of whenever I get to that point or dungeon.



ORIGNAL WALKTHROUGH:
This achievement unlocks after speaking to ArFA-sys back at home when their affinity rank is at least 4.

You can check your current rank with any of the character by pushing then go across to the player icon and click on friend list, you can then push and down the bottom on the right, it will say their rank along with a progress bar.

From what I can gather, the way you raise their affinity is by having that person in your party as your explore, completing quests, killing enemies both weak and strong all have a impact. Not sure if need but sometimes when out in the field and the characters activates ability's or gets double or triple kills using weapons, skills or gadgets, you can press to compliment them, not sure if this contributes but can't hurt to do that.