For the Set 'em Up, Knock 'em Down trophy/achievement you need to defeat a staggered enemy with a charged shot 15 times. Firstly you need to acquire the charged shot from a worktable, once you have it the easiest way to stagger enemies is using your element guns Ice or Lightning abilities to freeze or stun an enemy. Weaker enemies will freeze or stun when staggered but tougher enemies have a white bar under their health that you'll need to fill to stagger, you'll also need to get their health down quite a bit before staggering them as the charged shot might not take them out and you'll only get a few shots until the regain mobility. Once you have defeated 15 staggered enemies with charged shots, the trophy/achievement should unlock. It's likely that you will also unlock the Stagger Swagger trophy/achievement for staggering 25 enemies during this if you haven't already.
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