I noticed a lot of people struggling with Juggernaut's tavern fight, and as someone who did beat her, I can understand why.
Juggernaut is one of the toughest challenges in the game and she has, if I'm not mistaken, around 3-4 phases, with each phase giving her more and more attacks per turn, the highest being her final phase which only triggers at around 10% of her max HP being left, in which she has either 6-8 attacks per turn (I can't fully remember), but if you're someone who only reaches that phase and then immediately dies, I can see why you'd think she was unbeatable.
I give a lot of props to the dev Twisted Scarlet, I also used to think that there were forced losses in this game, only to go back and find out that for every boss that seemed impossible and scripted, could actually be beaten with the right preparations and strategies.
For me, what worked with SSJ was an Ant-shell counter build, which lets you evade all countered attacks, and then just tried to give my party as many counters per turn as possible.
I found that with a lot of bosses that you fight, the difficulty is not in damaging them-
The damage cap can be reached fairly easily in the mid-late game.
The difficulty is in surviving them and preparing for them-
And I think that makes the game very interesting to figure out and strategize for.