Yakuza 6: The Song of Life

This achievement unlocks automatically when you unlock the other 58 achievements for the game. You do NOT have to complete all the 285 awards listed for actions/statistical benchmarks you partake in the game itself, just to add for clarity.

As far as strategy goes, all but one of the achievements can be unlocked in your initial playthrough as well as the post-game Premium Adventure, with no restrictions on difficulty setting. The one and only leftover achievement that holds such restriction is the Dragon of Legend, where you have to complete a playthrough for the game on the Legend difficulty setting. The setting itself won't be available as an option until you complete your initial playthrough, so focus on completing everything to progress on the 57 achievements in the main campaign+Premium Adventure. Once you do everything and complete your initial run, create your post-game clear file, then select Continue in the main menu and select the file to start a new run, where you'll get the list of difficulty options with Legend now included.

I know that some on this forum commented that they unlocked the Dragon of Legend goal despite starting the game on a different difficulty and changing to Legend before proceeding to the final set of bosses (Koshimizu and then Tsuneo Iwami). But I couldn't see if this loophole was patched out or not, so I decided to just start my second playthrough on Legend from beginning to end, and I unlocked Dragon of Legend + The Song of Life upon completing it.

Just note, the key feature standing out in Legend difficulty is that attacks can take greater chunks of your life bar, especially from large groups of enemies as well as bosses. So before starting your second playthrough, head down to RIZAP and interact with the receptionist at the desk and purchase to the max count of 10 each one of the five supplements offered, the MUSCLE item will replenish your entire life bar no matter how low on health you are, and the other items can recover your health to up to 3/4 of the bar. Collectively it all comes to 760,000 yen, but you can make up for this through several runs in the Clan Creator minigame, especially the Part 3 side missions involving the Amon Army in either Kamurocho or Onomichi. Each successful run will reward you 200,000 yen.