Lucky for you, this is an easy achievement. Unlucky for you, you aren't going to enjoy grinding it.
Okay, so here's the idea. You have to choose a job and level it up from 1 all the way to 5. There are a couple of jobs you can pick from. Each has a small bit of variety but for the most part they are very simple games requiring a little timing and the A button. You can choose from a blacksmithing job, a woodcutting job, and being a bartender. Pick your poison, all of them are about as entertaining as reading the dictionary.
I personally found bartending to be the easiest despite the other solutions posted here. Check job postings regularly by going to the start menu and clicking on "Quests/Jobs." Usually an in game pop-up will appear as well. When your favorite job becomes available, just start grinding the hell out of it. Don't bother to sleep, eat, or do anything else in game. Just sit there and feel your brain slowly melt.
The trick to making this go faster of course is to "chain." That is, to serve drinks, cut wood, or smith swords perfectly, one after another. This is not difficult. The timing is not very picky, but after a while you are likely to become a zombie and screw things up on accident, probably once you've gotten a ridiculously high chain. Messing things up will set your multiplier back to the very beginning. If this happens to you, I have found that taking a small break from it can prevent your controller from being hurled through your television. However be wary of taking too long a break because the job may not be available after you go questing. A way to avoid this is to quit the job, pause the game, and do something non-video game related for a couple minutes. Then return to it.
I would also suggest muting the T.V while you grind this achievement. The barkeep praising your skill gets old about the twenty thousandth time, not to mention the very odd and distracting comments random passerby's will blurt out.