Gimli will be usable for this achievement once you form the fellowship in Rivendell. Throughout the story, you will have to throw Gimli to destroy out of reach cracked LEGO bricks. Opportunities for this are somewhat rare however, and you will need to grind this achievement.
To do it in a matter of a couple minutes, simply choose two tall characters directly after forming the fellowship. (Aragorn, Legolas, Boromir, and Gandalf are all options at this point).
Stand these characters a short distance from each other, and have one character grab Gimli with . Throw him to the other character using and click to switch to that other character. Then simply rinse and repeat, playing catch with the dwarf until the achievement unlocks.
Don't wait until beating the game to do this, because in Middle Earth you'll be restricted to two characters, and won't be able to toss Gimli back and forth. If you for some reason didn't get it earlier and you've beaten the story, you'll need to go into a story mode chapter with two tall characters and Gimli; the chapter 'Track Hobbits' works beautifully for this. This achievement is also unlocked quicker in co-op because there's no need to switch characters; you can also both work on it at once this way.