I won't be using spoiler tags, so it's advisable to only read this AFTER you've completed the game once.
THIS TROPHY CAN ONLY BE EARNT ON NG+, even if you do the requitements for another trophy that you will earn by doing this.
Step 1: Saving Private Ryo
Ryo is a ginormous pain in the rear end. You must get his affinity up to 50+ to save his life. I'm not sure if it can be done on lower but speaking from my own experience, it seemed to only trigger at 50. Once he survives Renamon's baiting by unconventional means. He will in turn help another character survive.
Step 2: Saving Corporal Shuuji
I found that Shuuji had to have a decent affinity too for Ryo to save him, however, I haven't been able to come up with a ball park figure, around 35 is the lowest I've had it work so far.
Step 3: Chapter 8 choice
DO NOT CHOOSE "Stay with Agumon" as your choice at the portal, as there is still a big big big chance Saki will die as part of the storyline, it's much better to never use replies with an Wrathful undertone. I chose to follow an harmonious personality set of choices to get this.
Step 4: Make sure you have level 70+ Digimon available
Unless you're playing on very easy during battles, you'll need some level 70 Digimon, It's advisable that two of these are Falcomon and Syakomon, as the God forms they get are OP in the final battle.