Fuga: Melodies of Steel

This is a time consuming achievement so I recommend saving it for last. I unlocked it on my third playthrough during chapter seven, using new game plus two times. Ideally, you'll already have some characters at or near level 50 by the time you focus solely on leveling up. While you may be tempted to take the most difficult paths on your second or third playthrough to maximize XP against enemies, I actually recommend taking the easiest paths and focusing on sleeping in the dormitory and doing expeditions to get multiple characters experience at a quicker pace.

With a fully upgraded dormitory, you can spend 5 AP to let 6 characters get 3,000 XP each. If you spend all of your AP on sleeping you will get 12,000 XP for each character during intermissions and most stages have three intermissions. At level 49 I needed a bit more than 50,000 XP to get a level, so it may take 4-5 intermissions of doing nothing but sleeping to get a level. But if you take the easy paths you'll reach intermissions fairly quickly.

If you have fewer than 6 characters to level up, you can instead opt to spend 5 AP doing the hardest expedition, the level 3 forge god one. It will earn you roughly 5,000 XP for 3 characters provided you earn an S rank on it. The expedition is a bit of a maze though, requiring you to pay attention and not waste ammo shooting down the wrong walls. Because it forces you to walk back and forth through the dungeon multiple times, I only did this to boost characters who were significantly under-leveled. Like the dormitory, it can be done 4 times per intermission.

The achievement unlocked for me after I got the last character to level 50 after a battle and had exited the victory screen.