The Eidolan is an Outsider. A villain with spellcasting can Dismiss it.
The issue with most classes is that they peak at different levels.
By "Peak" I don't mean they hit their most powerful point, I mean they hit their most powerful point when compared the the campaign, the enemies they are facing, and compared to their fellow party members.
Fighters peak really early and start to drop off for example.
Full casters peak much later, and Sorcerers and Wizards don't really have a real peak as they just keep getting better and better.
Synthesis Summoners peak at about level 9-10.
At this point, they are the most powerful they will be when compared to everything else leveling around them.
After this point, their increases in powerlevel aren't as incrementally unbalancing as they are during the 6-10 levels.
So, the good news is there is light at the end of the tunnel.
I'd suggest you try the dismissal thing. It's as easy spell for a villain to have level 10 or 11 and it shuts of the Eidolan.
Now I wouldn't suggest doing this on a regular basis, because you make the player feel their their character is useless if you just shut him down all the time.
It's a balancing act.
I'm currently running a game with a Sacred Shield Paladin in it, and his AC can hit well over 40 against a single evil target (like most Bosses of chapters) and he's only level 9.
So I feel your pain, but the answer to him is even more simple...combat maneuvers rule his world.
No PC is invincible, and the NPCs at your disposal are often far more capable then you initially give them credit for. They have all sorts of abilities that aren't explicitly written on their sheet, but are instead implied as things anyone can do. The problem with running such things is that we rarely run NPCs as well as we would our own sheets...just takes practice.
