I got 2 hard ones in Lego Star Wars II. Those are the ones where you have to fly around and you cannot use a "ghost" character to make it super-easy for you. You actually have to do it without getting killed. Hard stuff, took multiple tries.
Then I went and got 2 relatively easy ones in Lego Star Wars TCS. To get the "Arcade master" I went the easy route - fired up the arcade battle with 2 controllers, and used controller 2 to jump off the ledge 100 times - meaning the controller 1 got 100 "kills" and the achievement unlocked. "Cash in" was also easy when you know, in what mission and where to do it. :)
In the end of day played Wheelman and got 4 achievements during normal game progression - no grinding involved.