Resonance of Fate

You need to defeat at least 2 enemies in a single attack (does not apply to the tutorial).

There are several ways to do this. There is of course the grenade, use it on two enemies close together. Another way is during a heroic action jump over two close enemies and open fire on them. Correct timing along with moderate gun charge levels and lowered enemy HP can conspire to grant this to you.

A possibility that I have not tested is to kill an exploding enemy and their blast could take out a neighbor.

You can get this in Chapter One: Boss Fight vs Tar Man. At the start of the round there are 2 explosive spiders on top of the ramp. Just let one guy do Hero Action and jump to get on top of the ramp and then throw a grenade or do direct damage on them. They are very close together so just killing one should trigger a chain reaction getting the other spider.