The eidolon stat block you provided would be considered pathetic in most groups, there are builds at first level that can do double that amount of damage per round and still aren't considered overpowered. I'm not talking about optimized builds, I'm talking about builds that are considered to be normal. It's not the eidolon that's overpowered, it's that the majority of the party is exceptionally below the curve. If you honestly feel like the party balance is being threatened then sit down with the group and allocate attribute bonuses to those that you think need it the most to keep up with the summoner, you can just increase the CR a little bit if necessary as well.
does anyone else get flashbacks when they read these necrothreads? I was going through the early part thinking gosh this sounds familiar..
Anyhow if i remember correctly, one of the origonal issues from the ops game was it was a low fantsy game. when you have pet classes this creates a problem.
Essentially, Druid/eidolon pets advance assuming the party gets normal wealth progression. Which creates a power issue in a low fantasy game where the dm is holding back loot.
but really that eidolon was totally fine any even poorly built fighter following normal ideas for fighters should be better.
Specifically, they were running a Low Fantasy campaign with nerfed gear, and Summoners are one of those classes that are at least partially gear-independent. They'd have had the same problems with a Monk or Sorcerer.