This one is super easy. Once you get into a fight with a bunch of enemies and you start attacking, you will build up a 'combo' which is denoted by a x2, x3, x4 as you defeat enemies. Once you get to about a x3, a symbol of the B button should appear signifying that you can execute a finishing move. Hit B and you will execute the finishing move and the achievement will pop.
Also note that you get a boost to your Lego stud total for executing the move.
I have included a youtube video (all credit to them for the vid), that shows the solution on the first level.
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