Dusk Diver 2

MISSABLE

You'll learn about counterattacks during the Prologue/beginning combat tutorial. When you see the flashing red warning sign that their special attack is about to happen, this is what's called a "Crimson Strike" (see image below), and it's essentially the game telling you to counter this attack. Just before the "Crimson Strike" hits you, press  to perform a counterattack.
 

(Click images to enlarge.)

Admittedly, counterattacks are slightly more difficult to pull off than Just Dodges, even though their windows are quite similar; the ease somewhat depends on the attack itself and its animation. Once you've practiced it a few times though, it gets easier.

In all likelihood, like the majority of the combat trophies, you'll get this naturally as you play and shouldn't need to worry about it. If for whatever reason you don't use counterattacks often and you need to grind a few for the trophy, the Possessed people around Ximending and other Human Realm areas never stop spawning, and there's always loads of them around, so you can easily use them to farm encounters for this, or any other combat-related trophy.

Note: Despite how the tutorial shown in the image above describes it, the "Crimson Strike" is not a specific attack. This Crimson Strike is solely referring to the bright red warning sign that is shown before the attack takes place. Each enemy's Crimson Strike-attack is unique, and some are wildly different from others. Each enemy has only 1 Crimson Strike attack, and that attack stays the same for the entire game. In other words, only one of an enemy's different move sets can be countered.

For examples of the different Crimson Strikes, one robot's CS is spinning his arms in rapid windmills; one Kunlun fighter jet-looking enemy's flies straight into you; and one Chaos Beast's is just striking you with his claws.