In my experience, demons like Aeras, Python and any others that have "Becomes more powerful until link succeeds" in their linking are vastly more difficult than any other demons to recruit, to an unfair degree. I think this is in large part due to the 200% defense bonus that Heroism grants. I think this is bad for the game for a number of reasons:
-Recruiting these demons is dependent on party composition to a degree much greater than most other forms of recruitment. It's virtually impossible to deal enough damage in X number of turns unless you have two attackers that can target the demon's weaknesses, or one attacked that can hit weakness and some really strong attackers.
-The punishment for failing these links in high. The linked-with demon keeps their Heroism buff if you fail with them, which means you still have to beat a super-tough enemy on top of any other penalties for failing the link.
-A number of demons are almost never used. I'll admit that I don't have any statistics to back this up, but I've almost always failed at these links when I've tried them, and it's gotten to the point where I don't even try to recruit them unless I have a couple weakness-hitting demons.
-I've had situations where I've been able to beat heroism-buffed demons without much trouble; but that's because I had strong defenses. Python comes to mind; I've had it happen to me a couple of times where I've put in the work of getting rid of his minions first, then putting out a weakness-hitting demon, a healer, and another heavy hitter, and tried the recruitment... and failed because he takes so little damage from my attacks that I can't come close to beating him in 10 turns, even though I'm not in any danger of dying. After the link fails, I then began the slow process of whittling him down, and eventually beating him without ever being in any major danger. Situations like these are really frustrating and boring, and I feel like they represent the abnormally high difficulty of "beat this Heroism-buffed demon" links.