This trophy is one of the more difficult ones to get, because once you get above 29 Vault Dwellers, it opens up a greater variety of "incidents" that can show up, be it Mole Rats or Deathclaws. I ended up getting every other trophy first, then exiling various Dwellers to get under the 29 threshold.
One tip to note, however, is if you have a pet that gives "3x (or 5x) objectives", you can equip it to a Dweller in the Vault Entrance, and as long as the Raiders don't get past that room, you get the multiplier. I had given it a shot, and the number that shows up on the stats page still only increases by 1 each time, but the trophy unlocked for me at 37 Raider attacks.