As mentioned in DrEldarion's solution, the ideal way to get this trophy is by healing pedestrians as you play through the game.
If you resort to grinding this trophy, here's a good strategy:
- Run (don't take train) between medical centers near you, healing pedestrians you see on the roads as you head there.
- If you're lucky there will be a group of 3+ pedestrians at each medical center to heal, this can up your count fast. Make sure not to miss hospital beds if they are there.
- It will still take time but you should get this trophy before it feels too grindy.
Good luck!