Sacred Citadel

The earliest we can unlock this achievement is in Act 1, The Mine level. Here is where the game gave us the ability to uppercut an enemy, after we leveled up out character a second time.

To uppercut an enemy, press:
, +

There are other variations like:
, , +
, , , +
*You need to land the first hit or the sequence will not result in an uppercut*

To unlock the achievement wait for the game to give you the uppercut move and keep progressing through the level until you reach the part where a TNT explosion occur and more enemies come to attack. Now you should have 4 to 5 enemies in front of you. Just keep rolling until you have them all in a group and press uppercut them!

If you are having a hard time grouping the enemies you can do normal combos ending in so the final blow will launch the enemy to the side with the other enemies.

Also, you can unlock the achievement even earlier but since the level has some mine carts passing and hitting you and the enemies it makes it more difficult to keep them alive and together.

If you continue to the level and reach the boss you must restart the level cause you can't uppercut the boss.

To uppercut an enemy, do any of the following combinations:

x, y + left stick up
x, x, y + left stick up
x, x, x, y + left stick up

The real trick is getting 4 enemies on the screen at once (not a common occurrence), getting them all on the same level (plane) as you, and getting them close enough to perform the uppercut.

Once there are 4 enemies on the screen, I found it easier to smack them all to one side the screen using the [x, y + left stick left] or [x, y + left stick right] depending which side of the screen you wanted to go with. Once you smacked 4+ enemies onto one side of the screen, simply walk/sprint up to them and perform the uppercut combo and hope it hits at least 4 enemies. If it doesn't land, keep trying. A lot of the enemies roll away when they get back up from being knocked down, so you may need to set it up again if you miss.

Be patient... I wasn't able to land this achievement until the last stages of Act 4 (though, I'm sure with a bit of luck and good timing, you could get this much earlier).