I experimented today with a level 50 to try an S rank. It died in less than 5 seconds due to the overwhelming number of level 70s attacking at once and I got only 10 allied seals despite me hitting it from the start.
They need to be made harder. I know they're there for level 50s but here's the reality, level 50s aren't getting any or very little benefit and the level 70s aren't giving them a chance. Most the time it's only level 70s showing up to them.
I don't know how much more I can emphasize the necessity for harder S rank fights. There's no point in keeping them easy when lower levels aren't even bothering to show up and would rather hit level 70 first.