To "Just Guard" you hit B just before an enemy strikes you. At times, I found it helpful to lightly move LS toward the enemy and hit B at the same time, but this sometimes has a tendency to either dodge (you'll "scoot" behind the enemy) or dodge burst (things go black and white and slow-mo and you get in a bunch of rapid X's when prompted). So best thing is to just stand around in the middle of enemies, hitting B a bunch, or if trying to be more mindful of tactics, trying to hit B right as they're about to hit you.
You'll know it worked if you either (A) catch their attack and push them back, or (B) throw an attacker that was behind you over your shoulder, hence the title of the achievement. Note that this move can be powered up in the "skills" menu with a move that lets you hit "Y" immediately after a Just Guard, to perform a powerful follow-up attack. I found this useful at times, but isn't needed at all for the achievement.
I got this by playing the challenge mission "Trick Challenge Part 1" a few times.
To unlock the mission, during story Mission 4, look for a wall that looks damaged/cracked. It should be pretty obvious, and it's in one of the rooms near the beginning of the level. You may have to bust some pots or boxes first, to see it though. Switch to the "drill" sub-weapon and bust the door. The nurse is inside. Talk to her with "A" to unlock the challenge.
I got the drill sub-weapon from finishing "Mondo Girls 1" from the mission select menu. If you haven't seen it, it's basically this mini-game where you're a weird creeper trying to ogle the girl while she looks away without being noticed, occasionally giving gifts when prompted. I gave "Plain Chewing Gum" the first time, followed by "Single Flower" the second time. Gifts are purchased from the Office menu "Gift Shop".
Anyway, you get to "Trick Challenge Part 1" by selecting it from the nurse, after entering "Mondo Girls 2" from the Mission Select menu. I did a lot of punching with Y (which is what the challenge calls for) and dodging away with B (which I would fail, and get "Just Guard" instead). I failed the level several times but restarted, and the achievement unlocked after a few minutes of this, without even intentionally trying.
There is also a challenge you can unlock later that involves doing this very move several times in a row. But it's easier to grind out in a more open challenge.