This trophy is gained by reviving 20 allies during battle. This does not have to be in one battle but is cumulative over the span of all playthroughs (on the same save file that is).
Depending on your play-style, # of playthroughs, & difficulty this could pop naturally over the course of play.
Play Style - If you play more risky more people are likely to die (and can be revived).
# of playthroughs - If you decide not to focus on shares the first playthrough Neptune (or the other goddesses) will likely die several times.
Difficulty - If you play on normal or Hard cheap shots will likely kill any of the characters a few times during the game.
For most people who are trying to avoid an entire second playthrough (the game is over 50 hours long, so it makes sense) this will not come naturally.
The best way to get this trophy if not gained naturally is to wait until you are in-between the normal ending and true ending. Make sure Compa is level 50+ as she will need to be able to withstand hits and stay alive the entire time. Put Compa, your goddess of choice (mine was neptune) and IF on the front line in your formation.
First, stock up on all of the 4 different types of materials (gelatin, reflex, etc.). Make sure you have well over 400 of each type before heading into the dungeon.
Second, go into your characters skills. Lower any healing skills from *Goddess*/IF to 0%. In fact, lower every skill so that way nothing crazy can happen. Now crank up the self healing abilities and the lowest level revival skill for Compa. That way she does not die herself & only revives her fallen comrades for a small amount of health (easier for them to die again).
Third, change the equipment of *Goddess*/IF to the most basic ones available (if you still have them the starter set, otherwise buy 2 pairs of the lowest available in the story). That way they take more damage.
Fourth, crank the difficulty up to hard. That way you take more damage.
Fifth, choose a dungeon that is between level 30-50. The mobs should hit for between 500-800 damage per hit (lower is fine, it will just take longer to get *Goddess*/IF killed), but no more than 1k damage per hit (don't want Compa to die from a cheap shot). Run around until you find a group of 2-3 mobs (if you get a solo mob kill it quick and try again).
Sixth, have every character continually defend.
The trophy will pop shortly after the 20th person is revived. You do not need to end the battle to gain the trophy