Most of the Trophies in 13 Sentinels: Aegis Rim will be earned simply by playing the story through to completion. There are a few exceptions to this, and the In One Fell Swoop Trophy is one of them. Thankfully, this Trophy is easy to earn fairly early in the game.
You’ll need to have moved beyond The Beginning of the End section of the game for this, as I don’t think you’ll have a good opportunity to get a big enough cluster of enemies to pull this off.
The first stage you can earn this Trophy on (if I’m wrong about this, let me know in the comments) is 1st Area Ashitaba City Wave 0002. Here, you’ll be faced with absolutely massive swarms of tiny enemies (another good stage to try is 1st Area Ashitaba City Wave 0008, which has even bigger swarms). When you see a huge, dense cluster of enemies, use the Multi Rocket Launchers Skill, which has an enormous attack radius.
If you’re having trouble one-shotting these enemies, you’ll want to enhance the Multi Rocket Launchers Skill skill before battle, because the base level attack power might not be enough to get the full kill on all the enemies in the circle. With the upgrade, you should get it just fine.
And, if you’re struggling after upgrading, you can always reduce the difficulty to Casual, which reduces enemy health.
And that’s really all there is to it. If you can get 150 kills within a single blast radius, you’ll pop the In One Fell Swoop Trophy in 13 Sentinels: Aegis Rim.
