Children of Morta

There are six total playable characters. Each time you unlock a new character, I'd recommend rerunning the 1st dungeon (Caeldippo Caves, Silk Caverns) using the new character since I find it has the easiest boss.
This also ensures you don't have to keep track of which characters have beaten bosses, and which haven't.
I highly recommend having a couple points in Armor Up and Warfare (Workshop) to make your runs easier.
Achievement tracking DID work for me.

The description for this is kind of misleading. You actually only have to kill a boss with each of the six family characters. There is a seventh non-family character (the far left shadow on the character select screen) you eventually unlock, but I popped both the "unlock all playable characters" and "kill a boss with each character" achievements before even meeting this seventh person. Not sure if they were added with a game update and that's why the achievements don't count them or what.

Also important to note is that you don't have to kill different bosses each time. This is a huge relief if there's anyone you haven't levelled or generally find to be weak and hard to use (*cough*Linda). You can just go back to the first dungeon and fight the spider boss over and over again. Theoretically this is the easiest boss since it's the first level, but it can get tough if you don't pick off the secondary enemies fast enough and they start building up a lot and overwhelm you. Another good option is actually 2-3, Ruins of Old Barahut. I found that boss quite easy because it's slow, easy to see the attacks coming so you can dodge them, and doesn't spawn any secondary enemies to distract you. (Plus I got tons of health potion drops during the fight, but I don't know how much of that was just lucky RNG.)