Scope it out.
In every game with PVP you will get matches you will lose and matches you will win. If your MMR is high then the game thinks you are very good at the game. If it is low then it thinks that you are not.
Here is the twist.
MMR, high or low, doesn't affect how well you can play the game. It doesn't affect your character's stats. Your MMR can be sky high but it doesn't mean you are skillful or leveled up enough to handle that content.
If your MMR is 20-30% and you are running into level 150+ antagonists then, yeah, that is unfair. But, that doesn't ever happen to me when I am at 20-30% MMR.
If you are at 40-50% MMR and you are running into level 150+ antagonists then, yeah, that is fair. You are telling the game you can handle the bottom 40-50% of players with a reliable chance of victory.
It takes very little effort to get up to 40% MMR and only slightly more effort to get to 50%. Because the MMR increases even when you complete non-PVP content. Even if you are dying left and right and get carried, it will increase if the team wins. This is the biggest problem. Because it takes people who have not acquired enough skill or experience into the higher difficulties.
Since people are egocentric they will tend to believe they got to that MMR fair and square and then are baffled when they are stomped into the ground by a high level antagonist. This happens a lot. The problem is that people fail to grasp the meaning of the MMR system. How it can be manipulated and what it means to have high and low percentages.
Also, I see this a lot and I have to say; level 400 is as meaningless as level 200. All it means is that they played the game a lot which, sure, could mean they are better players but they are still constrained by MMR which will not take into account their player level, at least not in any significant way. Your passive stats max out at level 150 and you get your last Forge level at 199. Someone being level 700000000000 doesn't make them any stronger than a level 199.
My point is no one in this thread has said what MMR and what player level they are so it is impossible to determine if it was unfair or not. But at MMRs above 40% you should be expecting a difficult time ahead, mentor match or not.