So, that party didn't even have a proper tank? Or did they become overconfident that they got a proper healer?
Either way, they need, at least, a second front-line fighter to regain their balance.
@Mr-Stark - MC doesn't have much DPS, since he doesn't hit enemies much. But the strikes he does, do good damage. Probably he specializes in targeting weak points or some other fighting style that takes setup time.
@Reqqles - very realistic.
You can say, they hit a level where they need to read guides, organize in guilds, and otherwise start thinking, but their party doesn't have a person who is good at thinking. Stereotypically, a mage or a healer would do that, but their party was sorely lacking in brains.