The Force Unleashed 2 is a fairly short game. According to "How Long to Beat" it clocks in at around 8-16hrs depending on whether or not you're a completionist.
This achievement is unlocked by beating the game on medium/normal difficulty, obviously. Its story related and cannot be missed unless you play it on easy or simply don't finish the game. This should be your standard difficulty unless you're just breezing through the game on Easy for the story. It'd be recommended that if you're going for the difficulty based achievements you do your first play through on hard and a second on the "Unleashed" difficulty.
Like always, if you're not interested in following along in the story and immersion of the game or just want to get it done to boost your gamerscore... IGN has some wonderful guides.
https://www.ign.com/wikis/star-wars-the-force-unleashed-ii