SWORD ART ONLINE: FATAL BULLET

For this achievement, you need to gain a few affinity levels with all these characters...
Premiere
Strea
Philia
Rain

This achievement unlocked for me after getting them to affinity level 2. There is a separate achievement for getting all friends to affinity level 4 so just get everyone up to 4 and your obtain this achievement along the way. Make sure to always check to see if there is a green speech bubble after leveling a affinity level.

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

The fastest ways to gain points with the people in party is by reviving them.

On the Extreme difficulty, the snipers one shot everyone. Using the Photon swords skill Sword Barrier you can swoop in and revive them.

On the Normal difficulty the way I've been grinding this out is by fighting the Honeycombed enemy in Solitary Sand (Large mountain to the right of the map). This will require luck and being good at dodging.

The alternative solution is take on quests and go farming monsters. This will also help to level up your characters.