This is actually a very simple/easy silver trophy, and will be among the first bunch of trophies you get.
What the game means by "An army's worth" is actually 100 enemies... Simply defeat 100 total enemies and this silver is yours, if you don't skip all the battles you come across (it's wise to do them all, will make you more prepared for stronger foes) you should easily get this in Episode 1 without an issue.
Note: if you don't pause during these battles and hit the "Retry" option, you should have the
trophy by the time you get this trophy too, or little after.