Mysterious Figure is kind of fake hard compared to LW.
All you have to do to beat Mysterious Figure is use the right command (Thunder Surge) at the right time, and dodge for the rest of the battle. It's long, and it's easy to firetruck up, but with persistence it's very doable.
They both share a random pattern, it's just that you deal with Mysterious Figure the same way every time, whereas with LW you have to constantly be adjusting your strategy to manage the battle.
I can beat Mysterious Figure at Level 30. I'm at Level 99 and I've been trying off and on for weeks and I've yet to beat Lingering Will, and it's because no matter how you look at it, Lingering Will is really really firetrucking hard.
His pattern is random, some of his moves are instant kills and are really firetrucking hard to avoid, and even the best strategies are prone to failure if he decides to switch up which moves he uses. Openings are few and they can turn on you, and you get regularly eviscerated. It's on a different level entirely from Mysterious Figure.
TL;DR: Anyone can beat Mysterious Figure if they know the pattern. You have to actually be good at the game to beat Lingering Will.