Tanis a synthesis grapple build.
Get him like a trillion tails & tail slaps. Then it's grab evolution, greater grapple and you're golden. Maybe an Ursula type.
APG or Unchained?
Apg
Slam Build
Level 8 Eidolon Biped
Feats:Dirty Fighting,Improved Grapple,Improved Natural Attack(Slam)
Evolutions 11pts: Slam 1pt, Improved Damage(Slam) 1pt, Grab(Slam) 2pt, Reach(Slam) 2pt, Large 4pt, Push(Slam) 1
Being large with a reach slam, means that you can attack enemies 15 feet away from your large size reach, which lets you cover a big area. So, if you want to be more defensive with your eidoon, they can use the push evolution to push a foe backwards and then take a step back, so the enemy must move through their space again to get to the eidolon and provoke another attack of opportunity. And your bonus on grapple would be +4 if flanking, +2 for improved grapple, +1 for large, +4 for grab evolution and +6 BAB; for a total of +17 on grapple checks.
Your DM may decide that Improved Natural Attack and Improved Damage don't stack, but the Improved Damage evolution doesn't declare itself to be a size increase, so it should stack as written. If you can't get it, go with Combat Reflexes.
Evolution Priority:
Biped base form.
Bite evo twice for 1-1/2 str damage.
Max out str. Huge>large>ability increase evo. All ability increases from level-up in str.
Reach evo on bite.
Increased damage: bite
Spend remaining evo points on natural armor and/or climb speed
Feats: Power Attack, Furious Focus, Weapon Focus: Bite, Improved Natural Attack: Bite
Step 1: Summoner lock down target with battlefield control spells
Step 2: Then the squirrel ate Dib's greasy head
I probably should have mentioned some of the other nuances of the grapple slam build. You can choose to keep the claws from biped and put them on your feet instead of your hands, if you really want extra attacks. At level 9 you will gain multi attack and be able to attack twice with the slam if you don't have the claws though. And, if you only have 1 primary attack it does 1.5 x strength for damage.
Grab is really great. Against smaller enemies, it can allow you to grapple as part of your attack of opportunity, which can really ruin the action that provoked the attack. But even against larger enemies that +4 bonus it grants is great for a regular non-grab grapple attempt.
And one big attack is all you can use when maintaining a grapple, so extra attacks aren't that desired. And the same grapple limitation works as a bit of a defense against any enemy that relies on multiple attacks or two handed weapons.