Gotham Knights

All bosses take literally an hour to fight. The difficulty level does not affect obtaining the achievement, so it is not necessary to obtain equipment of suitable power, we just set the difficulty level to a very low level. At a low level, it will be enough to simply approach point blank and hit with a melee attack . After defeating each boss, you will receive recipes for crafting equipment of a higher level (45, 50, 55, etc.) and legendary materials; after each victory over the boss, you can craft more powerful equipment if you need it.

To start Showdown, go to the table behind the Batman memorial (to the right of the bike) in Balfrey and launch any boss. I hit in this order: regular Frieze, Harley, Clayface, Talia, Mecha Frieze (he's the stuffiest, so last).