| deanruel |
This is actually absurdly simple in that there's really nothing different you would do for a grapple Eidolon that you wouldn't do for all of them.
Make them Huge, buff their strength till you hover around 50, take grab, take lots of claw attacks, take Rake.
So by level 20 you're looking at something like 6 Claw attacks at +40 to hit or so, each one granting you a free grapple attempt, and each of those grapple attempts grants you 2 free rake attacks.
CMB would be something above +40 and the kicker is with every attack you get a new try. So if you don't succeed by the first you make your second attack and so on. Really unless you miss all 6 attacks or all six grapple rolls you're locking people down. Eidolons are just murderkings.